Do not wrap database errors, let django handle them.

Database error is a kind of error that requires our full attention
anyway, and it makes the site useless. There is no point in wrapping
them in a dozen places and replacing with a custom error handler which
gives small 'database error'.

Instead, let's just give django the full exception and handle those
exceptions properly. This will also make debugging tests a lot easier
since test failures will have the proper exceptions rather than hidden
failures.