Give rustfmt spawn error context.

[aDotInTheVoid]

This mean if you misconfigure to

{
    "rust-analyzer.rustfmt.overrideCommand": [
        "./nonono"
    ]
}

The error message is

[Error - 17:54:33] Request textDocument/formatting failed.
  Message: Failed to spawn "./nonono"
  Code: -32603 

instead of

[Error - 17:56:12] Request textDocument/formatting failed.
  Message: No such file or directory (os error 2)
  Code: -32603 

I'm not sure how to test this, or if it needs a test.

[jonas-schievink]

Thanks!

bors r+

[bors]

Build succeeded:

• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript (ubuntu-latest)
• TypeScript (windows-latest)

[lnicola]

changelog fix (first contribution) set error context when spawning rustfmt fails