Talking errors in casting failures

[davidepaolotua]

Implements #893. There is a breaking change, though. Previously, calling FailedCast#value would always return a nil value, without providing any clue on which value was actually breaking.
Now, it raises a FailCastError with a talking error message, in the form of - for instance - "Value ->Invalid<- is not a valid constant for enum WhateverEnum".

Btw, is there an "easy" way to test avram modifications in a dummy lucky app?

[jwoertink]

Hmm... I'm not sure about this one 🤔 The FailedCast was never intended to be an error. It's meant to just be a type check that it didn't know how to convert. By setting these values to nil, then your operation can handle the errors as you need. With this raising now, then you would never get to your operation in a full Lucky app. It would fail out before you had a chance to gather up all of the validation errors. And since exceptions are meant to be handled in your Lucky app's Error::Show action, this would end up breaking some of that convention.

I do see why you went this route though. I think I want to sit on this a little longer and see if there's an alternate way we can fix the issue without making some breaking changes here. Thanks for getting this started!