downgrade css identifier error from FATAL to WARNING

for some reason, a CSS selector like "input:-moz-placeholder" is 
a fatal error in es53, and it aborts processing.

I don't see a reason for this to be a fatal eror, so I'm downgrading
it to a warning. It's safe to do that because the warning still
marks that css rule as invalid, and it gets removed later.

[ihab.awad, 2013-06-17 20:58:11]

lgtm

[felix8a, 2013-06-18 04:22:53]

@r5452