core+gui: name the real cause of a failed build instead of guessing

The Building Will report showed a fixed list of three "possible reasons"
whenever a build produced nothing, regardless of what actually happened; in a
case reproduced from the owner log all three were false and the real cause was
not even listed. The "Checking your will" row had the same flaw, showing one
sentence ("Found CHANGES to the DATE or the HEIRS") for five situations,
including one where it is plainly wrong (funds received).

core/heirs.py: record WHY buildTransactions gave up in a new last_build_error
attribute (8 reason codes), set at each path that previously returned empty
with no explanation, plus a processed_willexecutors counter to tell "every
will-executor was skipped" apart from "we tried and failed". Also fix a latent
crash in the prepare_transactions handler, which read a no-longer-existing
e.heirname attribute and re-raised the resulting AttributeError, masking the
real error.

gui/qt/dialogs.py: add msg_alert() (amber warning sign, body text in the theme
colour, readable in both themes), _build_failure_message() and
_check_failure_message() to turn those causes into one precise sentence each,
with an honest "cause could not be determined" fallback. Catch
BalanceTooLowException, which already carried the figures but fell through to
the generic red technical error.

No new exception classes were introduced (owner request): the plain
NotCompleteWillException cases are told apart structurally, not by text.

CHANGELOG.md

@@ -2908,3 +2908,88 @@ renumber the grid rows.
 - Full test suite: 438 passed (unchanged).
 
 **Outcome:** DONE.
+
+---
+
+## 57. Name the real cause of a failed build instead of guessing
+
+**Date:** 2026-09-04
+
+**Goal (owner request):** the "Building Will" report was hard to read and often
+misleading.
+
+1. The long "could not build the will" block was printed entirely in amber
+   (`COLOR_WARNING`), which the owner reported as barely legible.
+2. Whenever the build produced nothing, the dialog printed a FIXED list of
+   three "possible reasons" (low balance / dust shares / check-alive later than
+   the delivery date) regardless of what had actually happened. In a case
+   reproduced from the owner's log all three were false, and the real cause
+   (no delivery date left to build) was not even in the list.
+3. The "Checking your will" row had the same problem: the single sentence
+   "Found CHANGES to the DATE or the HEIRS" was shown for five different
+   situations, including one where it is plainly wrong - funds received, where
+   neither the date nor the heirs changed.
+
+**What changed:**
+
+- `bal/core/heirs.py`
+  - `Heirs.__init__` / `buildTransactions`: new `last_build_error` attribute
+    recording WHY a build produced no transaction. Reset at the start of every
+    build, and set at each path that previously returned empty with no
+    explanation at all: `NO_HEIRS`, `NO_UTXO`, `NO_WILLEXECUTOR_USABLE`,
+    `NO_FUTURE_DATE`, `WILLEXECUTOR_FEE`, `WILLEXECUTOR_FEE_TOO_HIGH`,
+    `TX_BUILD_FAILED`, `WILLEXECUTOR_TX_ERROR`.
+  - Added a `processed_willexecutors` counter so that "the loop skipped every
+    will-executor" - which returned silently, with no log line whatsoever - is
+    told apart from "we tried and the build failed".
+  - Fixed a latent crash in the `prepare_transactions` exception handler. It
+    read `e.heirname` in order to auto-deselect the offending will-executor,
+    but NOTHING in the plugin sets that attribute any more (leftover from an
+    older exception design), so the lookup itself raised AttributeError and the
+    inner `except Exception: raise` re-raised THAT, aborting the whole build
+    with a confusing secondary error instead of the real one. The handler now
+    records `WILLEXECUTOR_TX_ERROR`, logs the actual exception together with
+    the will-executor it happened on, and moves on to the next one - which is
+    what the original code was clearly trying to do.
+
+- `bal/gui/qt/dialogs.py`
+  - New `msg_alert()`: an amber warning sign (U+26A0, written as a numeric HTML
+    entity so the source stays ASCII) followed by text in the theme's default
+    colour. Colour is what ATTRACTS attention, not what is read, so it is kept
+    on the sign alone; the message body stays readable and still works under
+    the dark theme, where a hard-coded black would disappear.
+  - New `_build_failure_message()`: maps `last_build_error` to ONE specific
+    sentence. When the code is missing or unrecognised it SAYS the cause could
+    not be determined and lists what to check, instead of asserting three
+    guesses as if they were the only possibilities.
+  - New `_check_failure_message()`: replaces the single "Found CHANGES to the
+    DATE or the HEIRS" line with seven precise messages, reusing the detail the
+    exceptions already carry (heir name, will-executor URL, old and new fee
+    rate). The two plain `NotCompleteWillException` cases are told apart
+    STRUCTURALLY (raised with no argument vs. with one), not by matching
+    message text, which would be fragile. No new exception classes were added
+    (owner request).
+  - Added a dedicated `except BalanceTooLowException` handler. The exception
+    already carried the balance, the fees and the dust threshold, but was
+    falling through to the generic handler, which printed the raw technical
+    string in red and re-raised. It now shows the real figures.
+  - "Checking variables" row: `No Heirs` now uses `msg_alert()`. The
+    "Check Alive Threshold Passed" message deliberately STAYS red
+    (`COLOR_ERROR`) because it is the more urgent situation (owner request).
+
+- `bal/gui/qt/common.py`
+  - Re-export `BalanceTooLowException` from `core.heirs` so the Qt layer can
+    catch it.
+
+**Verification:**
+- `py_compile` clean on all 44 files of the package.
+- The real `msg_alert`, `_build_failure_message` and `_check_failure_message`
+  were extracted from the source via AST and executed against every reason code
+  and every exception type, with the exception hierarchy rebuilt from
+  `will.py`: 9 build cases and 8 check cases all produce the intended text.
+- NOT RUN: the official test suite. The machine used for this task (Windows)
+  has no importable `electrum` module, so `tests/` could not be executed.
+- Manually tested by the owner in Electrum 4.8.1: `NO_FUTURE_DATE`,
+  `WILLEXECUTOR_FEE` and `No Heirs` were all confirmed on screen.
+
+**Outcome:** DONE.