Improve the error reporting of svn_repos_verify_fs3 with KEEP_GOING off
if no notification callback has been provided, i.e. when looking for
the first corruption issue.

* subversion/libsvn_repos/dump.c
  (svn_repos_verify_fs3): Make the detected error part of the error chain
                          returned when it did not get reported, yet.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1631557 13f79535-47bb-0310-9956-ffa450edef68