Improve overall quality of compile_error! errors

Also updates UI tests.

test_suite/tests/ui/default-attribute/enum.stderr

@@ -1,8 +1,8 @@
 error: #[serde(default)] can only be used on structs with named fields
- --> $DIR/enum.rs:4:10
+ --> $DIR/enum.rs:6:1
   |
-4 | #[derive(Deserialize)]
-  |          ^^^^^^^^^^^
+6 | enum E {
+  | ^^^^
 
 error: aborting due to previous error
 

test_suite/tests/ui/default-attribute/nameless_struct_fields.stderr

@@ -1,8 +1,8 @@
 error: #[serde(default)] can only be used on structs with named fields
- --> $DIR/nameless_struct_fields.rs:4:10
+ --> $DIR/nameless_struct_fields.rs:6:9
   |
-4 | #[derive(Deserialize)]
-  |          ^^^^^^^^^^^
+6 | struct T(u8, u8);
+  |         ^^^^^^^^
 
 error: aborting due to previous error