An invalid category previously resulted in either a blank page (with the error indicated in the title) or an incorrectly formed feed. It is almost never meaningful to die over an invalid category when generating a title.
Tweak warning message for unexpected_not_comparison
Rename "unexpected_not_for_in" to "unexpected_not_in".
Respect escaping within regular expressions.
Fix the useless assignment warning.
Fix traceback when dumping loops.
Split into two separate warnings.
Rename warning for clarity; add test.