Add a ResponseException type that can be converted to a Response

This would provide a way to allow failures to have a default representation that the backend can render if the service doesn't handle it. This will work well for things like decoders where the user doesn't typically want to deal with a parsing exception, but the client should be notified that there was a problem related to it.
A second evolution. I think I like it better.
More refinements of ReplyException
Merge branch 'release-0.4' of https://github.com/http4s/http4s into release-0.4
Merge branch 'release-0.4' of https://github.com/http4s/http4s into release-0.4