In 'svnadmin verify --keep-going', don't treat a user-cancellation
as a normal error that could be ignored.

* subversion/libsvn_repos/dump.c
  (svn_repos_verify_fs3): Leave if the error is actually a cancel request.
                          Also, report errors found during the metadata
                          verification phase.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1559872 13f79535-47bb-0310-9956-ffa450edef68