Use httplib constants instead of hardcoded error values.

Simple change to remove the raw HTTP error codes and replace them with
httplib's constants.

Testing done: I've been using rbt post / post-review.

Reviewed at https://reviews.reviewboard.org/r/4776/
Clean up the git diff call a little bit

Sponsored-by: Zonar Systems, Inc
Signed-off-by: Garrett Cooper <yanegomi@gmail.com>
Merge remote-tracking branch 'upstream/master'

Conflicts:
	rbtools/postreview.py