Don't panic on broken symlinks when listing toolchains

[mkrasnitski]

I have a stage1 symlink in my toolchain cache that can be broken depending on the state of my local copy of the rustc repository. Running cache cache toolchain while this symlink is broken panics in the following way:

thread 'main' panicked at src/commands/toolchains.rs:62:27:
called `Result::unwrap()` on an `Err` value: Error { depth: 0, inner: Io { path: Some("/home/michael/.local/share/rustup/toolchains/stage1"), err: Os { code: 2, kind: NotFound, message: "No such file or directory" } } }

It would be great to either print a proper error in this case, or otherwise continue.