Do not log CSS parse errors to stderr by default.

Fix #2083. Change the log level of CSS errors from error to info.
Remove a workaround for a Rust bug.

We’ve upgraded to a Rust version that fixed mozilla/rust#13322.
Optimize CSS error logging: check log level before task-local silencing.
auto merge of #2445 : SimonSapin/servo/css-warnings, r=jdm

This is on top of the #2433 Rust upgrade.