qml: show swap service problem to user

electrum/gui/qml/components/MessageDialog.qml

@@ -17,6 +17,7 @@ ElDialog {
 
     parent: Overlay.overlay
     modal: true
+    z: 1 // raise z so it also covers dialogs using overlay as parent
 
     anchors.centerIn: parent
 

electrum/gui/qml/qeswaphelper.py

@@ -184,6 +184,7 @@ class QESwapHelper(AuthMixin, QObject):
             asyncio.run(swap_manager.get_pairs())
             self._service_available = True
         except Exception as e:
+            self.error.emit(_('Swap service unavailable'))
             self._logger.error(f'could not get pairs for swap: {repr(e)}')
             return