installing nightly with missing components suggests doing 'rustup component remove'

[jethrogb]

When trying to install a toolchain that's missing components you may get an error message, e.g.:

$ rustup toolchain add nightly-2019-07-23
info: syncing channel updates for 'nightly-2019-07-23-x86_64-unknown-linux-gnu'
info: latest update on 2019-07-23, rust version 1.38.0-nightly (e649e9034 2019-07-22)
error: component 'clippy' for target 'x86_64-unknown-linux-gnu' is unavailable for download for channel nightly-2019-07-23
Sometimes not all components are available in any given nightly. If you don't need the component, you can remove it with:

    rustup component remove --toolchain nightly-2019-07-23 --target x86_64-unknown-linux-gnu clippy

The suggested action is useless, however:

$ rustup component remove --toolchain nightly-2019-07-23 --target x86_64-unknown-linux-gnu clippy
error: toolchain 'nightly-2019-07-23-x86_64-unknown-linux-gnu' is not installed

The correct suggestion would be to run the original command again with --profile minimal

[rbtcollins]

This occurs because we use the same codepath for add toolchain and add component, and the error isn't contingent on the context.

[Rustin170506]

I am working on this.