Return messages

[aldesantis]

I'm not sure how this could work, but it would probably be very useful if allowed users to return an error message when forbidding.

I imagine it could look something like this:

def update?
  return true if user.admin? || resource.author == user
  [false, 'You must be an admin or the author to update an article.']
end

# somewhere else
result, error = policy.update?

The only semantic issue with this proposal is that predicate methods are supposed to return a boolean, while this would make them return true or an array.