rustic-doc assumes the existence and use of rustup

[therealpxc]

On some projects, I use Nix to provide my Rust toolchain, since I generally like to use Nix to provide all of my 'external' (i.e., when it comes to Rust projects, not supplied by Cargo), per-project dependencies.

I still can do documentation lookups at point thanks to rust-analyzer, which is great, but I seem to be without docs search, which sucks. It'd be nice if rustic-doc could work for setups with and without rustup, and at the very least it should give some kind of error message instead of just choking.

[psibi]

Sounds good, PRs welcome.