Update 'fsfsfixer' to work with svnadmin v1.7+.

* contrib/server-side/fsfsfixer/fixer/fix-rev.py
  (handle_one_error): Ignore the extra error message shown by svnadmin v1.7+.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1554945 13f79535-47bb-0310-9956-ffa450edef68