Format on save does not format if `rustfmt` has internal error

rustfmt will sometimes output internal errors, even when it successfully formats the code. This happens particularly frequently when using let-else. If you happen to write some code with a trailing whitespace in a let-else block, then rustfmt will format everything else in the file and output an error about not removing trailing whitespace.

Rust-analyzer will ignore rustfmt's output if there is any sort of error, even if rustfmt actually formats the document. This means that some documents will randomly become unformattable,

I think the way to solve this is to look to see if there are any errors that aren't marked with the [internal] marker and ignore the error if all errors are [internal].

Reproducer: (note the whitespace after the else)

fn main() {
let opt = Some("hello world");
    let Some(a) = opt else { 
        return;
    };
}

With RA_LOG set to warn, this produces the following output:

[WARN rust_analyzer::handlers] rustfmt exited with status 1 command="rustfmt" "--edition" "2021" captured_stderr=error[internal]: left behind trailing whitespace
 --> <stdin>:3:3:29
  |
3 |     let Some(a) = opt else { 
  |                             ^
  |

warning: rustfmt has failed to format. See previous 1 errors.

Relevant code: https://github.com/rust-lang/rust-analyzer/blob/f85fc285088129a131ecb83398d9f66922e0830e/crates/rust-analyzer/src/handlers.rs#L1871

rust-analyzer version: rust-analyzer version: 0.3.1472-standalone (01120f121 2023-04-07)

rustc version: rustc 1.69.0-nightly (001a77fac 2023-01-30)

Start in crates/rust-analyzer/src/handlers.rs around the rustfmt handling at line 1871, and reproduce the let-else example with RA_LOG set to warn. Check how rustfmt stderr and exit status are interpreted; done means formatting is accepted when all reported errors are marked [internal], while other errors still prevent applying the output.