feat(willexecutors): differentiate empty server response from network error

Raise NoServersForChainError when the welist server responds but returns
no data for the requested chain, showing a chain-specific message instead
of the generic network error.

bal/core/willexecutors.py

@@ -73,6 +73,10 @@ chainname = BalPlugin.chainname
 
 class Willexecutors:
 
+    class NoServersForChainError(Exception):
+        """Raised when the welist server responds but returns no data for the
+        requested chain, indicating no active servers for that network."""
+
     # Expose the networking constants as class attributes so the GUI layer can
     # reference them (e.g. to show the "Xs / DEADLINEs" countdown) without
     # importing module-level names.  Single source of truth: the module

bal/gui/qt/window.py

@@ -1112,6 +1112,7 @@ class BalWindow:
                 candidates.append(url)
 
         result = {}
+        any_server_reached = False
         net = Network.get_instance()
         _logger.info(f"fetch_will_executors_list: network present = {net is not None}")
         for url in candidates:
@@ -1124,6 +1125,7 @@ class BalWindow:
                 resp = Willexecutors.send_request(
                     "get", url, timeout=10, max_retries=1, retry_sleep=1,
                 )
+                any_server_reached = True
                 _logger.info(
                     f"fetch_will_executors_list: resp type={type(resp).__name__} "
                     f"len={len(resp) if hasattr(resp, '__len__') else 'n/a'}"
@@ -1142,6 +1144,8 @@ class BalWindow:
                 _logger.error(
                     f"fetch_will_executors_list: {url} -> {type(e).__name__}: {e}"
                 )
+        if not result and any_server_reached:
+            raise Willexecutors.NoServersForChainError(chainname)
         return result
 
     # Simple, user-facing message shown when the download fails for any reason
@@ -1205,6 +1209,13 @@ class BalWindow:
 
         def on_failure(exc_info):
             _logger.error(f"download_list failed: {exc_info}")
+            if isinstance(exc_info[1], Willexecutors.NoServersForChainError):
+                chainname = BalPlugin.chainname
+                self.show_warning(_(
+                    f"No active will-executor found for the "
+                    f"{chainname} network."
+                ))
+            else:
                 self.show_warning(_(self.DOWNLOAD_FAILED_MESSAGE))
 
         self.waiting_dialog = BalWaitingDialog(