Improve overall quality of compile_error! errors

Also updates UI tests.

test_suite/tests/ui/enum-representation/internal-tuple-variant.stderr

@@ -1,8 +1,8 @@
 error: #[serde(tag = "...")] cannot be used with tuple variants
- --> $DIR/internal-tuple-variant.rs:4:10
+ --> $DIR/internal-tuple-variant.rs:7:5
   |
-4 | #[derive(Serialize)]
-  |          ^^^^^^^^^
+7 |     Tuple(u8, u8),
+  |     ^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

test_suite/tests/ui/enum-representation/internally-tagged-struct.stderr

@@ -1,8 +1,8 @@
 error: #[serde(tag = "...")] can only be used on enums
- --> $DIR/internally-tagged-struct.rs:4:10
+ --> $DIR/internally-tagged-struct.rs:6:1
   |
-4 | #[derive(Serialize)]
-  |          ^^^^^^^^^
+6 | struct S;
+  | ^^^^^^
 
 error: aborting due to previous error
 

test_suite/tests/ui/enum-representation/untagged-and-internal.stderr

@@ -1,8 +1,14 @@
 error: enum cannot be both untagged and internally tagged
- --> $DIR/untagged-and-internal.rs:4:10
+ --> $DIR/untagged-and-internal.rs:5:9
   |
-4 | #[derive(Serialize)]
-  |          ^^^^^^^^^
+5 | #[serde(untagged)]
+  |         ^^^^^^^^
 
-error: aborting due to previous error
+error: enum cannot be both untagged and internally tagged
+ --> $DIR/untagged-and-internal.rs:6:9
+  |
+6 | #[serde(tag = "type")]
+  |         ^^^
+
+error: aborting due to 2 previous errors
 

test_suite/tests/ui/enum-representation/untagged-struct.stderr

@@ -1,8 +1,8 @@
 error: #[serde(untagged)] can only be used on enums
- --> $DIR/untagged-struct.rs:4:10
+ --> $DIR/untagged-struct.rs:6:1
   |
-4 | #[derive(Serialize)]
-  |          ^^^^^^^^^
+6 | struct S;
+  | ^^^^^^
 
 error: aborting due to previous error