Service Transport sendRequest implementations hide errors

[stewSquared]

When calling a service through a serviceTransport (implementations in HttpRestServiceTransport and TestRouteServiceTransport), failures aren't propogated back to the client in with complete information. This is troublesome for any endpoint that might 4xx with information that should be passed to the client rather than wrapped in a 500. In particular, this is inconvenient for any delegating endpoints.

[stewSquared]

Please see "Note" in this akka-http documentation: https://doc.akka.io/docs/akka-http/current/routing-dsl/exception-handling.html

Related PRs and discussions:
#73
https://github.com/drivergroup/driver-test-utils/pull/10#discussion_r148386055
https://github.com/drivergroup/driver-core/pull/73/files#r148316153

cc @zachdriver @vuspenskiy, who've discussed this with me previously.