Make rbt and post-review consistent in their treatment of missing passwords.

The way we handled the http response wasn't the same between the postreview.py
and api/request.py. In particular, different improvements had been made to each
without porting to the other. The rbt version checked the validity of
'response', while the post-review one used httplib's constants. This change
makes the two consistent.

Based on https://reviews.reviewboard.org/r/4787/