In the fsfsfixer contrib script, tweak the usage message and and error
message for clarity.

* contrib/server-side/fsfsfixer/fixer/fix-rev.py
  (fix_one_error): Clarify that the real problem is the script doesn't
    recognize the error message.

* contrib/server-side/fsfsfixer/fix-repo
  (USAGE): Clarify.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1477721 13f79535-47bb-0310-9956-ffa450edef68