Fixed panic when no toolchain was configured

[Glamhoth]

Fixed panic when no toolchain was configured and user run
rustup default.

[kinnison]

Can you describe the situation in which this happens, and would it be possible to write a test to demonstrate it no longer does? If the testing is too hard I won't complain too much, though with the introduction of a message, it'd be good to have a test which explicitly trips it.

[Glamhoth]

It happens when there are no toolchain configured in .rustup/settings.toml file, i.e. just after rustup was installed. I think it would be possible to test if running rustup default returns, in that case, an error.

[kinnison]

OK, yes I can now reproduce the issue so I agree it needs fixing. That's good.

Now we need to devise a test for this.

Perhaps, in test/cli-misc.rs you might try adding:

#[test]
fn no_panic_on_default_missing() {
    setup(&|config| {
        expect_err(
            config,
            &["rustup", "default"],
            "no default toolchain configured",
        );
    })
}

And see if that helps. I can confirm that the above test would panic on current master.

[Glamhoth]

Test passes after my fix.

[kinnison]

LGTM