Switch all uses of 422 response code to 400.

The 422 response is normally only for WebDAV, and these types of
errors should normally return a 400 code, HTTPBadRequest. This
commit goes through all the remaining 422 codes and changes them
to 400 responses.

Fixes bug 1153807

Change-Id: I9bf49643763c2cb0ac585641056c85a060152e36
Merge "Switch all uses of 422 response code to 400."