If there's inconsistent state in the database due to hand-modifications

or some form of corruption, it's possible for the diff viewer to attempt

to load comment fragments into containers that don't exist. If this

happens, the diff viewer completely breaks. It really shouldn't happen

outside of development setups, but when it does, it's bad.
This change looks for this and skips the comment, leaving an error in

the console and moving on.

Loaded several diffs: Working ones, and a broken one that prompted this

change. All loaded successfully, and the broken one notified about the

broken containers and comment IDs.
Unit tests pass.