Improve overall quality of compile_error! errors

Also updates UI tests.

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

@@ -1,8 +1,13 @@
 error: variant `Struct` cannot have both #[serde(serialize_with)] and a field `f1` marked with #[serde(skip_serializing)]
- --> $DIR/skip_ser_struct_field.rs:4:10
-  |
-4 | #[derive(Serialize)]
-  |          ^^^^^^^^^
+  --> $DIR/skip_ser_struct_field.rs:6:5
+   |
+6  | /     #[serde(serialize_with = "serialize_some_other_variant")]
+7  | |     Struct {
+8  | |         #[serde(skip_serializing)]
+9  | |         f1: String,
+10 | |         f2: u8,
+11 | |     },
+   | |_____^
 
 error: aborting due to previous error