Expand error message for cases when extruders are disabled

It is also unable to slice models that are on extruders that are disabled.

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -411,7 +411,7 @@ class CuraEngineBackend(QObject, Backend):
 
         if job.getResult() == StartJobResult.NothingToSlice:
             if self._application.platformActivity:
-                self._error_message = Message(catalog.i18nc("@info:status", "Nothing to slice because none of the models fit the build volume. Please scale or rotate models to fit."),
+                self._error_message = Message(catalog.i18nc("@info:status", "Nothing to slice because none of the models fit the build volume or are assigned to a disabled extruder. Please scale or rotate models to fit, or enable an extruder."),
                                               title = catalog.i18nc("@info:title", "Unable to slice"))
                 self._error_message.show()
                 self.setState(BackendState.Error)