Hoist ParseErrorReporter into style and remove the dependency on msg.

The pipeline id stuff is currently unused. If someone needs it, they can add
an additional trait bound on their css error reporter to get the pipeline id.

components/style/context.rs

@@ -5,9 +5,9 @@
 use animation::Animation;
 use app_units::Au;
 use dom::OpaqueNode;
+use error_reporting::ParseErrorReporter;
 use euclid::Size2D;
 use matching::{ApplicableDeclarationsCache, StyleSharingCandidateCache};
-use msg::ParseErrorReporter;
 use selector_matching::Stylist;
 use std::cell::RefCell;
 use std::collections::HashMap;