Added WithErrorAs parameter

`napping` switches to unmarshal the return message as
`*napping.Request.Error` instead of `*napping.Request.Result`.

Added `WithErrorAs` to fill the `*napping.Request.Error` so user may still
get the error messages in those cases. Dynamically switches expectation
routines to test against the error (thats why error has to be a valid
`Response`).