On the verify-keep-going branch, tweak generation of the error summary.

This fixes a small layering violation, and also changes the output a bit.

* subversion/libsvn_repos/dump.c
  (notify_verification_error_summary): Don't populate notify->warning_str.
   The warning string is intended to be used for svn_repos_notify_warning,
   not for svn_repos_notify_failure_summary.
   Instead, we can rely on API users to obtain and display error messages
   based on notify->err. This was a small layering violation since API
   users might want to use a different string representation anyway.

* subversion/svnadmin/svnadmin.c
  (repos_notify_handler): Make 'svnadmin' generate an error message display
   based on the error chain saved in notify->err. Display all interesting
   (i.e. non-tracing) errors in the error chain saved for a revision.
   Also, pad the output so that error messages are aligned for revision
   numbers with up to 6 digits (TODO: padding could account for larger
   revision numbers, like 'svn blame' does).


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/verify-keep-going@1546041 13f79535-47bb-0310-9956-ffa450edef68