Add Error::is_not_found or similar

[jplatte]

Use case: Some automation tools require other binaries to be installed. I would like to catch when that is not the case and print installation instructions.

[matklad]

SGTM!

Should probably add a .kind() method which returns a fieldless nonexhaustive enum.

[matklad]

Thinking about the broader problem, I think for myself I usually use the following pattern:

if cmd!(sh, "my-tool --version").run().is_err() {
    eprintln!("Install my tool");
    return
}

That is perhaps a better behavior than catching the error after-the-fact, because:

• it's more transactional (we don't abort in the middle of the process)
• you also see a version of the tool printed to the screen, which helps with debugging issues when the tool exixsts, but it has wrong version (thinking about weird CI case mostly).

[matklad]

Revisiting, I lean towards not doing it:

• calling tool --version up-front is the right pattern to do this
• the error message xshell produces converts FileNotFound into command not found already