Bugnote '~' processing may produce error if bug does not exist

This error case is a bit far-fetched, and should normally not occur
unless there is some data corruption. If referencing an existing bugnote
whose parent bug is not in the database, the callback function cannot
retrieve the project_id, so error 403 is triggered.

The callback functions have also been reformatted for better readability

Fixes 0012580