Provide an exception interface all exceptions implement

Makes total sense to me, we have added a lot of exceptions since we started.

I'll set this to 1.2.x, since I can't see any reasons that this would be a breaking change?

I think we should add a follow up issue as well after that, to write documentation about the exceptions and document when to extend the marker interface or use the exceptions as a third party module.

@marcus_johansson,

Should the interface be named `AiExceptionInterface` ?