Remove 'Failed to load resource' message from rewrite_css filter.

[GoogleCodeExporter]

E.g.

[Wed Nov 10 16:59:57 2010] [error] http://es.jcamunasltrulala.com/55388:29:
Failed to load resource 
http://jcamunasltrulala.com/style/calendar_rt.css?v=0cb5fb21


This message should not go in the logs -- it's not indicating a failure.  It's 
just indicating that the resource has not yet been put into the mod_paggespeed 
server cache, so on this pass of the HTML it will not be optimized.

Original issue reported on code.google.com by jmara...@google.com on 10 Nov 2010 at 4:14