Improve error message when override doesn't exist

[est31]

Try this:

• rustup override add nightly-2016-11-02
• downloads specified nightly and sets it as override
• rustup toolchain remove nightly-2016-11-02
• removes toolchain
• rustup show

Gives output:

error: toolchain 'nightly-2016-11-02-x86_64-unknown-linux-gnu' is not installed
info: caused by: not a directory: '~/.multirust/toolchains/nightly-2016-11-02-x86_64-unknown-linux-gnu'

It can be fixed by removing the override but I think that should already happen when you do rustup toolchain remove

[brson]

Good find. Thank you.

[brson]

Hm, after thinking about this, I'm not sure that removing toolchains should remove overrides. Just because the toolchain doesn't exist doesn't invalidate the need for that toolchain to build a project. For now I'd prefer to just have better error messages here.

[brson]

Hm, this error basic error is not horrible, but is obscured by the 'cause', which is irrelevant here. A better error though would indicate that an override requires this toolchain but it is not installed, install it with ... etc.

[est31]

Or one could warn about present overrides when uninstalling.

[brson]

I think I prefer to solve this by simply making a better error message, like:

error: override toolchain 'nightly-2016-11-02-x86_64-unknown-linux-gnu' is not installed

And getting rid of the causal error info: caused by: not a directory: '~/.multirust/toolchains/nightly-2016-11-02-x86_64-unknown-linux-gnu', which is an implementation detail.

It would be even better if rustup suggested how to remove the override, but the current UI doesn't have a way to do that.

[shiver]

I took a quick stab at resolving this. The output should look something like this:

$ rustup show
Default host: x86_64-unknown-linux-gnu

error: override toolchain 'nightly-2016-11-02-x86_64-unknown-linux-gnu' is not installed

Hopefully that is what you were after.
Thanks!