Do not warn on 'Failed to create or read input resource'

[GoogleCodeExporter]

E.g.

 [Sat Nov 06 22:25:48 2010] [warn] Failed to create or read input resource /j15/templates/system/css/system.css

This is at best an "info" severity and probably could be eliminated entirely.  
It occurs in the combine_css filter whenever an input CSS file is not in the 
cache, either because this is the first time it's been seen, or because the 
input CSS file was not cacheable in the first place.

Original issue reported on code.google.com by jmara...@google.com on 8 Nov 2010 at 12:07