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_errors/src/lib.rs

@@ -1551,7 +1551,7 @@ impl DiagCtxtInner {
                     &mut out,
                     "delayed span bug: {}\n{}\n",
                     bug.inner
-                        .styled_message()
+                        .messages()
                         .iter()
                         .filter_map(|(msg, _)| msg.as_str())
                         .collect::<String>(),