`multirust default <toolchain>` doesn't handle manual uninstallation of a toolchain properly

``` shell
$ /Users/yberreby/.multirust/toolchains/nightly/lib/rustlib/uninstall.sh
...
$ multirust default nightly
multirust: using existing install for 'nightly'
multirust: default toolchain set to 'nightly'
$ rustc
/usr/local/bin/rustc: line 120: /Users/yberreby/.multirust/toolchains/nightly/bin/rustc: No such file or directory
/usr/local/bin/rustc: line 120: exec: /Users/yberreby/.multirust/toolchains/nightly/bin/rustc: cannot execute: No such file or directory
```

I'm pretty sure `multirust default nightly` should detect that the nightly toolchain is missing, and reinstall it, instead of happily reusing an existing install that doesn't actually exist.

Reproduce the sequence using toolchains/nightly/lib/rustlib/uninstall.sh, then run `multirust default nightly` and `rustc` to observe the stale-install behavior. Trace the `default` command's handling of an existing toolchain and define done as detecting the missing installation and reinstalling it instead of selecting the absent compiler.

Written by the indexing model from the issue text.