control-chars.py: defensive coding in the error path.

* tools/hook-scripts/control-chars.py
  (main): Gracefully handle error strings that don't match our regexp.
    (Not tested.)


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1461393 13f79535-47bb-0310-9956-ffa450edef68