During install, handle failure to install toolchain better

[brson]

Right now, we do the (very simple) rustup install, then install stable. If stable fails, then the installation is reported as failed, but rustup is still installed.

I'm not sure off hand if it's better to report a successful install with caveats or to try to revert the install completely.

Mentioned in #260

[peschkaj]

This came up in #rust-beginners yesterday. A user had installed rustup through rustup-setup.exe, but they had no working rust.

@mbrubeck suggested that the user run rustup default stable followed by rustup update stable - this resulted in a working installation.

Maybe we could keep rustup installed, but if stable fails, display a message to the user that something went wrong and prompt them to run those commands and report any errors as an issue.

[brson]

SGTM

[brson]

A special error message for the case where the stable toolchain doesn't exist could be helpful too.