Have user function return a Result

[sjwiesman]

Currently, user functions return an Effect which means in the case of failure they can only panic. If instead, it returned a Result<Effect, SomeErrorType> there would be a more graceful failure scenario.

[aljoscha]

Yes, this is a good point. We could either change the existing method or add a new one.

Also, I'm reworking error handling a bit: #9

[slinkydeveloper]

This is something that may be handled in #12. The macro, from the return type, can figure out if there's any need to handle the Result