Improve import result message interface

## Feature requests on the [forum](https://forum.activitywatch.net/t/message-internal-server-error-when-i-try-import-buckets/507/4)

**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.