Modify NancyEngine to allow hosting handler access to StatucCodeHandlers and to make use of consistent exception response handling.

Note that the FindStatusCodeHandler changes effectively incorporate PR #1688 (wasn't sure how to pull it in and modify). I'm not sure if this is a breaking change or not.
The two new methods have not (yet) been added to INancyEngine to avoid any further breaking changes. Ideally these should be added to the interface contract.
Modify EndProcessRequest to accept INancyEngine, use  with improved error handling.

The changes hopefully avoid being breaking by adding INancyEngine as an optional paramter and taking advantage of the issue raised in #1549.
Add call to ToList() on relevantHandlers variable to avoid test error.