Implement Try for ExitStatus

[fenhl]

It would be useful if std::process::ExitStatus implemented std::ops::Try. This could reduce boilerplate code where multiple Commands need to be spawned and checked.

Open question: What's a good error type? ExitStatus itself already contains all the relevant error information from the command, but being able to return a result of Err(ExitStatus::from_code(0)) would be weird. (Thanks @scottmcm for pointing this out.)

[scottmcm]

Note that if the trait redesign from #42327 (comment) were to happen, a distinct error type would no longer be needed and thus this would become substantially more reasonable.

[luser]

For a real-world example of where this would be useful, sccache runs compiler commands and wants to exit with the same status as the compiler:
https://github.com/mozilla/sccache/blob/3f318a8675e4c3de4f5e8ab2d086189f2ae5f5cf/src/lib.rs#L71
https://github.com/mozilla/sccache/blob/3f318a8675e4c3de4f5e8ab2d086189f2ae5f5cf/src/commands.rs#L526

[ratmice]

I believe that this should be closed as per the libs-api discussion in #93565