Extend CommandError::ExecutionFailed to optionally include stderr

[syphar]

since rust-lang/docs.rs#2467 we are now also seeing pre-build errors in docs.rs.

Some of the errors need more details for crate authors to work with them, example issues:

• kube-custom-resources-rs failed to build docs.rs#2624
• several Bevy crates failed to build with yanked dependencies error but could not find which docs.rs#2556
• Missing targets when requested from metadata docs.rs#2424
• Build failure not recorded if failure during crate fetch step docs.rs#797

( there are more, but these examples should suffice )

With this change we now extend CommandError::ExecutionFailed and the necessary PrepareError variants with the captured stderr output, which should give crate authors all the information they need.

[GuillaumeGomez]

Nice improvement, thanks!