Improve some names.

Lots of vectors of messages called `message` or `msg`. This commit
pluralizes them.

Note that `emit_message_default` and `emit_messages_default` both
already existed, and both process a vector, so I renamed the former
`emit_messages_default_inner` because it's called by the latter.

compiler/rustc_resolve/src/late.rs

@@ -3693,7 +3693,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             let mut parent_err = this.r.into_struct_error(parent_err.span, parent_err.node);
 
             // overwrite all properties with the parent's error message
-            err.message = take(&mut parent_err.message);
+            err.messages = take(&mut parent_err.messages);
             err.code = take(&mut parent_err.code);
             swap(&mut err.span, &mut parent_err.span);
             err.children = take(&mut parent_err.children);