Show message when slicing failed with nonzero exit code

The slicing engine should never crash. If it does though, it's best to show something to the user. Otherwise the slicing process just halts and the user will wait a long time for it to never finish.

Contributes to issue CURA-6568.

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -4,12 +4,12 @@
 import argparse #To run the engine in debug mode if the front-end is in debug mode.
 from collections import defaultdict
 import os
-from PyQt5.QtCore import QObject, QTimer, pyqtSlot
+from PyQt5.QtCore import QObject, QTimer, QUrl, pyqtSlot
 import sys
 from time import time
 from typing import Any, cast, Dict, List, Optional, Set, TYPE_CHECKING
 
-from PyQt5.QtGui import QImage
+from PyQt5.QtGui import QDesktopServices, QImage
 
 from UM.Backend.Backend import Backend, BackendState
 from UM.Scene.SceneNode import SceneNode
@@ -922,9 +922,31 @@ class CuraEngineBackend(QObject, Backend):
 
         if not self._restart:
             if self._process: # type: ignore
-                Logger.log("d", "Backend quit with return code %s. Resetting process and socket.", self._process.wait()) # type: ignore
+                return_code = self._process.wait()
+                if return_code != 0:
+                    Logger.log("e", f"Backend exited abnormally with return code {return_code}!")
+                    message = Message(
+                        text = catalog.i18nc("@message", "Slicing failed with an unexpected error. Please consider reporting a bug on our issue tracker."),
+                        title = catalog.i18nc("@message:title", "Slicing failed")
+                    )
+                    message.addAction(
+                        action_id = "report_bug",
+                        name = catalog.i18nc("@message:button", "Report a bug"),
+                        description = catalog.i18nc("@message:description", "Report a bug on Ultimaker Cura's issue tracker."),
+                        icon = "[no_icon]"
+                    )
+                    message.actionTriggered.connect(self._reportBackendError)
+                    message.show()
+                else:
+                    Logger.log("d", "Backend finished slicing. Resetting process and socket.")
                 self._process = None # type: ignore
 
+    def _reportBackendError(self, _message_id: str, _action_id: str) -> None:
+        """
+        Triggered when the user wants to report an error in the back-end.
+        """
+        QDesktopServices.openUrl(QUrl("https://github.com/Ultimaker/Cura/issues/new/choose"))
+
     def _onGlobalStackChanged(self) -> None:
         """Called when the global container stack changes"""