(PUP-1549) Use standard reason phrases on webrick

The webrick interface was using the body of the response for the reason
phrase for non-2xx responses. In the v1 api, this meant that we got
bizarre response headers. In the v2 api, it meant that we got json
formatted body data in the first header.

Not only was that state of affairs odd, it was also inconsistent with
the rack implementation, which didn't ever set the reason phrase. I am
also a little concerned that some clients might have choked on large
reason phrases (header too long issues).

This removes the setting of the reason phrase on webrick, which give us
back the standard reason phrases.
(PUP-1549) Update documentation showing reason phrase

All of the error responses that we had used as examples in the API
documentation had been taken from using webrick. Now that webrick is
consistent with rack, the docs need to be updated to show what the
responses should really look like.
Merge pull request #2315 from zaphod42/issue/master/pup-1549-v2-funky-response-phrase

(PUP-1549) Use standard reason phrases on webrick