Improve import result message interface

Feature requests on the forum

Is your feature request related to a problem? Please describe. Upon successful import, a null message is displayed, which is very difficult to associate with a successful import. And if import fails, the message {"message": "Internal Server Error"} is displayed all the time, which also does not say what the problem is.

Describe the solution you'd like If import is successful, the message Import completed successfully! is displayed. And if import fails, a message is displayed that the import could not be completed, and the specific reason why (and also its solution is possible).

Describe alternatives you've considered

Additional context The server should give the correct error messages, which can be displayed in the web interface to the user.