Feature flag to get non-string error as value

[cheesycod]

Would be nice if mlua could offer a error-value feature flag to allow retrieving the value of the error directly instead of it being just stringified into a traceback. This would be useful for allowing tables (for example) to be used as errors from Luau (which i want to do for better error handling on my hobby project).

[Pspritechologist]

I actually made an Issue about this just last week funny enough. #631