* Introduce a few error types in `InwardResponse`
* Improve readability of ExternalServer in Federator by use of Polysemy.Error also in callLocal
* This also solves an existing TODO whereby federator integration tests didn't work due to InwardResponses, whether an error or an expected return value, were parsed always as InwardResponseBody. This may have been an issue with mu-haskell when parsing (only needed in tests), since the behaviour when using `grpccurl` was correct. This is now sidestepped by using more than a simple string on errors.

This PR is in preparation to sanitize request paths against path traversal attacks (separate PR https://github.com/wireapp/wire-server/pull/1646)