rename unexpected_try_recover function

[bb1yd]

While reading the code, the doc comment and the name of the function unexpected_try_recover confused me. The doc said it tries to recover if it is a closing delimiter but I see no recovering code and the function always return an error.

So I change the name of the function to unexpected_err and change the return value to Diag<'a>. I also update the doc comment.

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

[rustbot]

r? @JohnTitor

rustbot has assigned @JohnTitor.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler, parser
• compiler, parser expanded to 73 candidates
• Random selection from 17 candidates