Complement to debf521d.

Also removed try/except/fail antipattern that hides exceptions.

Refactored "Prevented unneeded index creation on MySQL-InnoDB" (2ceb10f3)
to avoid setting db_index=False.

Thanks Berker Peksag for the suggestion.

Added the AddIndex and RemoveIndex operations to use them in migrations.

Thanks markush, mjtamlyn, timgraham, and charettes for review and advice.

Thanks Tim Graham for the review.

Since 859fc020, this is no longer needed.

Thanks Tim Graham for the review.

Thanks Tim Graham for the initial patch.