Improve overall quality of compile_error! errors

Also updates UI tests.

test_suite/tests/ui/remote/bad_getter.stderr

@@ -1,8 +1,8 @@
 error: failed to parse path: "~~~"
-  --> $DIR/bad_getter.rs:10:10
+  --> $DIR/bad_getter.rs:13:22
    |
-10 | #[derive(Serialize)]
-   |          ^^^^^^^^^
+13 |     #[serde(getter = "~~~")]
+   |                      ^^^^^
 
 error: aborting due to previous error
 

test_suite/tests/ui/remote/bad_remote.stderr

@@ -1,8 +1,8 @@
 error: failed to parse path: "~~~"
-  --> $DIR/bad_remote.rs:10:10
+  --> $DIR/bad_remote.rs:11:18
    |
-10 | #[derive(Serialize)]
-   |          ^^^^^^^^^
+11 | #[serde(remote = "~~~")]
+   |                  ^^^^^
 
 error: aborting due to previous error
 

test_suite/tests/ui/remote/enum_getter.stderr

@@ -1,8 +1,14 @@
 error: #[serde(getter = "...")] is not allowed in an enum
-  --> $DIR/enum_getter.rs:10:10
+  --> $DIR/enum_getter.rs:11:1
    |
-10 | #[derive(Serialize)]
-   |          ^^^^^^^^^
+11 | / #[serde(remote = "remote::E")]
+12 | | pub enum E {
+13 | |     A {
+14 | |         #[serde(getter = "get_a")]
+15 | |         a: u8,
+16 | |     },
+17 | | }
+   | |_^
 
 error: aborting due to previous error
 

test_suite/tests/ui/remote/nonremote_getter.stderr

@@ -1,8 +1,11 @@
 error: #[serde(getter = "...")] can only be used in structs that have #[serde(remote = "...")]
- --> $DIR/nonremote_getter.rs:4:10
+ --> $DIR/nonremote_getter.rs:5:1
   |
-4 | #[derive(Serialize)]
-  |          ^^^^^^^^^
+5 | / struct S {
+6 | |     #[serde(getter = "S::get")]
+7 | |     a: u8,
+8 | | }
+  | |_^
 
 error: aborting due to previous error