Auto merge of #6656 - phansch:command-failed-print-stderr, r=flip1995
Auto merge of #6656 - phansch:command-failed-print-stderr, r=flip1995

clippy_dev: Pass stderr to CommandFailed

This improves error reporting when running `rustfmt` fails for some reason, as seen [on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/257328-clippy/topic/Issue.20with.20rustfmt). It will now include the stderr output in the `CliError::CommandFailed` error.

changelog: none

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust