The recent improvements to error reporting caused gtk_css_provider_load...
to always return TRUE and leave the error unset. Error messages were instead
dumped out with g_message, which is not helpful.

This commit changes things back to the way they were before:
If a GError is passed in, parsing will fail at the first error, reporting
it in the given GError. If no GError is passed in, we keep going and
just issue the warning messages. This fixes the parser tests.