Better error propagation

[marph91]

In an error case, the notification tells the user to check the developer console and look for the correct error message. For a regular user this is cumbersome.

Check which options we have to put meaningful error descriptions directly in the notifications or propagate them in another way to the user.

Reference: https://discourse.joplinapp.org/t/joplin-export-export-emails-from-thunderbird-to-joplin/24792/50