core: create error pathway in Envoy

Currently, Envoy Mobile maps direct responses generated by Envoy to its `onError` callback paths. In addition to using a fairly brittle mechanism to detect these local responses, this also engenders other shortcomings in the available context and manner in which errors are raised. Adding a more generic handle where envoy creates local responses, and allowing the call to create the response _or_ raise an error along an explicit error path would be a superior solution, allowing for better error context and avoiding risky assumptions.