res_ari: Make some additional error responses consistent with the rest of the system.

This change makes some error cases use ast_ari_response_error to construct their
error responses instead of manually doing it. This ensures they are consistent
with the other error responses.

Based on the original patch as done by Paul Belanger on the associated review.

Review: https://reviewboard.asterisk.org/r/2904/
........

Merged revisions 408957 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@408958 65c4cc65-6c06-0410-ace0-fbb531ad65f3