Improve error handling by bypassing ParseError and ShellError

[cassaundra]

Currently, in functions which are required to emit ShellErrors (such as in nushell Commands), we convert the internal diagnostic to a ShellError::GenericError, which is quite lossy. The same is the case for parse errors.

To circumvent this lossy conversion, we can push errors instead to the quake engine itself, and handle them independently of nushell engine errors.

[cassaundra]

Adding on to this: should refactor calls to exit inside of the engine with a EngineError result type.