Improve overall quality of compile_error! errors

Also updates UI tests.

test_suite/tests/ui/with-variant/skip_ser_whole_variant.stderr

@@ -1,8 +1,10 @@
 error: variant `Unit` cannot have both #[serde(serialize_with)] and #[serde(skip_serializing)]
- --> $DIR/skip_ser_whole_variant.rs:4:10
+ --> $DIR/skip_ser_whole_variant.rs:6:5
   |
-4 | #[derive(Serialize)]
-  |          ^^^^^^^^^
+6 | /     #[serde(serialize_with = "serialize_some_unit_variant")]
+7 | |     #[serde(skip_serializing)]
+8 | |     Unit,
+  | |________^
 
 error: aborting due to previous error