Give a more helpful error message for rustup which rustfmt if it's not installed #3321
Description
Problem you are trying to solve
These errors are inconsistent:
; rustup which rustfmt --toolchain beta
error: not a file: '/home/jyn/.local/lib/rustup/toolchains/beta-x86_64-unknown-linux-gnu/bin/rustfmt'
; rustfmt +beta
error: 'rustfmt' is not installed for the toolchain 'beta-x86_64-unknown-linux-gnu'
To install, run `rustup component add rustfmt --toolchain beta-x86_64-unknown-linux-gnu`
Solution you'd like
The second error here ('rustfmt not installed') seems more helpful; it would be nice to show that consistently.
Notes
; rustup --version
rustup 1.26.0 (5af9b9484 2023-04-05)
Note that this error message also happens for commands which aren't proxied:
; rustup which ls --toolchain beta
error: not a file: '/home/jyn/.local/lib/rustup/toolchains/beta-x86_64-unknown-linux-gnu/bin/ls'
I'm not sure the right thing to do here; maybe the current error is good enough?