Add missing return

This caused the slicing to not be correctly aborted when the primetower was
in the wrong location

CURA-8499

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -468,6 +468,7 @@ class CuraEngineBackend(QObject, Backend):
                 self._error_message.show()
                 self.setState(BackendState.Error)
                 self.backendError.emit(job)
+                return
             else:
                 self.setState(BackendState.NotStarted)