Emit annotated item in error message

[zhassan-aws]

The replacement of the unmaintained proc-macro-error crate with the proc-macro-error2 crate in #3493 resulted in a diff in the error message Kani emits for https://github.com/model-checking/kani/tree/main/tests/ui/derive-arbitrary/union.

With proc-macro-error, the error was:

error: Cannot derive `Arbitrary` for `Wrapper` union
 --> union.rs:5:10
  |
5 | #[derive(kani::Arbitrary)]
  |          ^^^^^^^^^^^^^^^
  |
note: `#[derive(Arbitrary)]` cannot be used for unions such as `Wrapper`
 --> union.rs:6:7
  |
6 | union Wrapper {
  |       ^^^^^^^
  = note: this error originates in the derive macro `kani::Arbitrary` (in Nightly builds, run with -Z macro-backtrace for more info)

With proc-macro-error2, the error is:

error: Cannot derive `Arbitrary` for `Wrapper` union
       
         = note: `#[derive(Arbitrary)]` cannot be used for unions such as `Wrapper`
       
 --> union.rs:5:10
  |
5 | #[derive(kani::Arbitrary)]
  |          ^^^^^^^^^^^^^^^
  |
  = note: this error originates in the derive macro `kani::Arbitrary` (in Nightly builds, run with -Z macro-backtrace for more info)

The two notable differences are:

1. The annotated item (union Wrapper) is no longer included in the error
2. The first note does not appear after the span.

We should investigate if it's possible to restore the previous behavior.

[zhassan-aws]

Created GnomedDev/proc-macro-error-2#1 to ask about the difference in output.