feat: more descriptive messages when no sources are selected to backup

[diivi]

fixes #1463

[real-yfprojects]

I think a change to create.py would be better suited.

[diivi]

I looked through create.py, but didn't understand how it can detect an error from borg.
The run method is responsible for executing a command, and only that method knows whether borg returned this error message.
This method is called from the JobsManager which should remain untouched.
So how can I parse the output that borg gives from a subclass of BorgJob, when only BorgJob actually runs the command? Thanks

[real-yfprojects]

While you can't catch the error in create.py you can foresee it. This is already done but not working correctly as we noticed:

vorta/src/vorta/borg/create.py

Lines 99 to 101 in e20e923

	if n_backup_folders == 0 and '--paths-from-command' not in extra_cmd_options:
	ret['message'] = trans_late('messages', 'Add some folders to back up first.')
	return ret

[real-yfprojects]

Great! This is exactly what I had in mind.