move error handling from libsyntax/diagnostics.rs to libsyntax/errors/*

Also split out emitters into their own module.

src/libsyntax/ext/expand.rs

@@ -1301,7 +1301,7 @@ pub fn expand_crate(mut cx: ExtCtxt,
 
         let mut ret = expander.fold_crate(c);
         ret.exported_macros = expander.cx.exported_macros.clone();
-        cx.parse_sess.span_diagnostic.handler().abort_if_errors();
+        cx.parse_sess.span_diagnostic.abort_if_errors();
         ret
     };
     return (ret, cx.syntax_env.names);