3.0 - Fix reading of nested errros

@markstory Wasn't sure about that, so I referred to https://github.com/cakephp/cakephp/pull/3928#discussion_r14766112. But you're right, testing against the interface would probably be better, as the entity doesn't necessarily have to extend `Entity`. On the other hand, the class consuming the entity trait doesn't necessarily have to implement the entity interface, but I'm not sure whether that is a valid use case in the first place.