Installation help message is printed when attempting to use uninstalled component for toolchains created with toolchain link

[camelid]

Problem
Currently if a component is not installed for a custom-linked toolchain, you will get an error message like this:

error: 'rustc' is not installed for the toolchain 'r2stage1'
To install, run `rustup component add rustc --toolchain r2stage1`

Yet that is not helpful since the toolchain is not an official toolchain that just doesn't have the component installed. Instead, the error should be something like:

error: 'rustc' is not installed for the toolchain 'r2stage1'
The toolchain 'r2stage' was created with `rustup toolchain link` so you will have to install the component yourself.

Steps

1. Link a toolchain; e.g.: rustup toolchain link stage1 rust/build/x86_64-apple-darwin/stage1
2. Delete a component (e.g. by running ./x.py clean with the example in (1))
3. Try using the component: rustc +r2stage1 foo.rs

Possible Solution(s)
I suggest an error like this:

error: 'rustc' is not installed for the toolchain 'r2stage1'
The toolchain 'r2stage' was created with `rustup toolchain link` so you will have to install the component yourself.

Notes

Output of rustup --version: rustup 1.22.1 (b01adbbc3 2020-07-08)

[camelid]

(Sorry for the not super clear title -- I wasn't sure how to describe the issue)

[kinnison]

I think this is related to #2470 which was fixed in #2487 and so will be in the next release.

I'm tagging this as such so that you'll know when we make the next release.

[camelid]

Oh, great! Thanks @jyn514 :)

[kinnison]

Rustup 1.23.0 is now out.