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.
Would it make more sense to re-throw the exception here if there is no match instead of a fallback handler?  If there was a fallback handler it might require checking the exception attachment on the exchange instead of handling exceptions normally in later code.  We could still have a fallback handler by default but I think you should be able to remove it if you do not want all exceptions to be caught.