ExceptionHandler

A HttpHandler that delegates to an underlying handler.  In the case of an exception during execution of the underlying HttpHandler dispatch to a different HttpHandler based on the Exception's class.

The order the exception handlers are added is important to handle inheritance.  I noticed some of the JAX-RS exception mappers try to calculate the inheritance depth to handle this.  I don't think it needs to try and be that clever.
No, I don't think we should. The attachment key should probably just go on the ExceptionHandler class for now, although I am thinking that at some point we may want an Attachments class for common attachment keys.