Error handling and shell execution

[thi4go]

Right now, hephaestus-shell isn't handling properly errors coming from the monad executions and the interactions with our pure code base. The ideal would be to model a type for each error class and return it properly.

Also, our shell panics when there's an unhandled error, which makes us loose our loaded environment. We need to find a way where it preserves the env and returns a human-readable error in a safe manner from the created error types.