Give a better error message when a component isn't installed for a custom toolchain

[jyn514]

Closes #2470.

I did the thing you told me not to do which is mention x.py :P let me know if I should remove it.

I'm not sure how to run a custom rustup without installing it on top of my existing install ... cargo run gives me

warning: it looks like you have an existing installation of Rust at:
warning: /home/joshua/.local/lib/cargo/bin
warning: rustup should not be installed alongside Rust. Please uninstall your existing Rust first.
warning: Otherwise you may have confusion unless you are careful with your PATH
warning: If you are sure that you want both rustup and your already installed Rust
warning: then please reply `y' or `yes' or set RUSTUP_INIT_SKIP_PATH_CHECK to yes
warning: or pass `-y' to ignore all ignorable checks.
error: cannot install while Rust is installed

Is there a way to test this by hand or do you only have unit tests? I want to see what the output looks like.

[kinnison]

Personally I do cargo run -- --no-modify-path -y but it sounds like you have an odd setup there.

I think CONTRIBUTING.md carries some instructions on setting up a custom cargo home.

[kinnison]

This is pretty close to spot on. I'll even forgive the x.py providing you tweak the wording a bit.

[kinnison]

Looks good, please rebase to remove the response commit now I've checked it.

[jyn514]

Rebased :)