Handle parse errors during diff migration.

The new diff size column would cause migrations for old diff data. In the case
where a very old review request has un-migrated data which doesn't parse
correctly (from back when you could attach a bogus diff to a review request),
it would cause an un-caught exception. This change handles DiffParseErrors in
the recalculate_line_counts method.

Testing done: Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/5485/