Improve overall quality of compile_error! errors

Also updates UI tests.

test_suite/tests/ui/duplicate-attribute/with-and-serialize-with.stderr

@@ -1,8 +1,8 @@
 error: duplicate serde attribute `serialize_with`
- --> $DIR/with-and-serialize-with.rs:4:10
+ --> $DIR/with-and-serialize-with.rs:6:25
   |
-4 | #[derive(Serialize)]
-  |          ^^^^^^^^^
+6 |     #[serde(with = "w", serialize_with = "s")]
+  |                         ^^^^^^^^^^^^^^
 
 error: aborting due to previous error