Unignore some svn_error_t * returns, via tools/dev/warn-ignored-err.sh.

* subversion/libsvn_ra_serf/serf.c (load_config, svn_ra_serf__open),
* subversion/libsvn_subr/prompt.c (prompt),
* subversion/tests/libsvn_subr/auth-test.c (cleanup_callback):
    Don't ignore errors.

* subversion/svn/svn.c
  (sub_main): Explicitly clear or handle errors from svn_cl__help().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1467261 13f79535-47bb-0310-9956-ffa450edef68