Rokit does not always gracefully handle invalid manifests

[filiptibell]

Rokit sometimes indexes directly into TOML documents, which may panic if no value exists for the key, example:

https://github.com/filiptibell/rokit/blob/d41b102f40cb7d58c083803d540d154a7e4ea645/lib/manifests/rokit.rs#L91-L94

self.document["tools"] will panic here if the user has manually written an empty Rokit manifest, accidentally deleted / renamed the tools section, etc.