Fix error type

[mikedilger]

Error type is currently hyper::Error, and as things stand upstream this is hard to move off of.

But we need to generalize this, and we need to do so in such a way that consumers of pemmican can extend the error type.

[mikedilger]

Consumers are no longer tied to hyper::Error. We map their error into a hyper::Error::Io using ErrorKind::Other. This is temporary until the following issue is resolved in hyper 0.12:
hyperium/hyper#1128

Handler functions should generally send 5xx error responses and return Ok

[mikedilger]

We no longer map back to a hyper error, but rather return InternalServerError to the client and send hyper a success. This is appropriate.