Log post-commit errors in the Apache error log as well as returning
them to the client.

* subversion/mod_dav_svn/version.c
  (dav_svn__checkin): Log APR_EGENERAL rather than serr->apr_err to
   avoid httpd giving "APR does not understand this error".
  (merge): Log any post-commit error.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606004 13f79535-47bb-0310-9956-ffa450edef68