qabro creates a new bug every time there is a permission issue on Launchpad

[Pierre Equoy]

Steps to reproduce:

1. Start qabro
2. Select a project you have the writes to create issues for, but not the right to edit series, status or importance (for instance, `qa-candidate`)
3. Modify the importance (e.g. select `Critical`)
4. Submit the issue by pressing Alt+Enter

→ You will see an error ("Error: " — this should be made more user-friendly), and in ~/snap/qabro/common/qabro.log you will see something like:

    User does not have sufficient permissions to edit the bug task importance.

→ However, a bug has already been created

5. Modify the importance back to `Undecided` and submit again
→ This time, no error reported and the bug is created with files attached.

The issue is that bugs are open for nothing (as many as the times it took the user to figure out what was wrong)

If a bug has been opened already, we should reuse it and try to edit its content, instead of creating a new one.