* Combine MultiVerb swaggers into a map

* Make ErrorDescription schemas inline

This fixes an issue with MultiVerb error responses all having the same
schema (the first one being declared).

* Merge MultiVerb schemas for error descriptions

* Reuse combineResponseSwagger in ErrorDescription

The combineResponseSwagger function, introduced for `MultiVerb`, can
also be used to implement the functionality of `CanThrow`. This PR
reimplements the `HasSwagger` instance of `CanThrow` on top of
`combineResponseSwagger`, leaving the functionality the same modulo the
order of responses.

* Update CHANGELOG

Co-authored-by: Akshay Mankar <akshay@wire.com>