Improve consistency of error reporting in 'svnauth' and 'svnrdump', mainly
by channeling nearly all errors thrown within sub_main() up to a single
handler in main().

This is a source code improvement: no functional change.

This is a follow-up to r1543805 which did the same for 'svn' and 'svnadmin'.

* subversion/svnauth/svnauth.c,
  subversion/svnrdump/svnrdump.c
  (EXIT_ERROR, SVN_INT_ERR): Remove.
  (sub_main): Let this return a standard Subversion error object, and add an
    exit_code output parameter for those cases where a non-zero exit code is
    needed without an error message.
  (main): Display any returned error here.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1543834 13f79535-47bb-0310-9956-ffa450edef68