Fix a couple of issues with PR#571

1. renderer validity checks were too stringent for plugins, format
   should only need to be checked when mode is 'xhtml'
2. E_USER_WARNING message should now be obsolete, its previous use
   was to generate a warning when 'xhtml' rendering failed. PR#571
   insures 'xhtml' rendering always succeeds.
remove test expectation for E_USER_WARNING
Merge pull request #614 from splitbrain/renderer_graceful_fallback_bugfix

Fix a couple of issues with PR#571