Cita should better handle corrupt note attachments

[diegodlh]

Is your feature request related to a problem? Please describe.
If the "Citations" note attachment is manually modified and becomes corrupt, the contents of the "Citations" pane is not rendered:

1. Add a citation to an item
2. Edit the "Citations" note attachment manually to introduce a syntax error
3. Open the "Citations" pane

Describe the solution you'd like
A note may be shown in the citations pane explaining what happened and giving options to fix the problem. For example: let the user manually fix the note attachment, or remove the corrupt attachment altogether.