Improve readability (and parsability) of less common commit errors
using ra serf by prefering the server generated error message text
over the raw error when possible.

This patch also drops a quite common double reporting of raw HTTP
errors, such as when committing to a file that is locked in a repository.

This makes the errors more common to those reported by the other ra
layers.

* subversion/libsvn_ra_serf/commit.c
  (return_response_err):


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1551927 13f79535-47bb-0310-9956-ffa450edef68