Yet another serf server response handling improvement.

* subversion/libsvn_ra_serf/commit.c
  (add_file): Improve diagnostics. Not every HTTP status code tells us that a
    node already exists. Use the usual error handling for other responses.
    Use the same error code as the other RA layers use.

* subversion/tests/cmdline/commit_tests.py
  (commit_add_file_twice): Apply stricter error check.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1526439 13f79535-47bb-0310-9956-ffa450edef68