Improvement to delegation exception

I’m hoping this is an appropriate place to get some feedback on a pending pull request I have on rails/rails: https://github.com/rails/rails/pull/10828

Copied from the PR:

I found my self in a situation today where I wanted to rescue from a delegate exception where the object I was delegating to was nil. Right now this throws a standard RuntimeError with a detailed message, which is great, but in this case I didn’t want to rescue from other exceptions. I think adding this simple DelegationError class would make working with these exceptions a bit cleaner.

I hope others find this valuable. Feedback welcome!