ra_local: use symbolic error names in the maintainer-mode warnings function.

* subversion/libsvn_ra_local/ra_plugin.c
  (ignore_warnings): Print the symbolic names of the rror that is
    being ignored.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1481823 13f79535-47bb-0310-9956-ffa450edef68