Cowboy sub protocol

I think I got this right but I am very unfamilar with Plug. Will need to confirm at least:

* Same error response is sent when a plug raises an exception
* Same response is sent when no response is sent by plug

TODO:
* Automatically add the translator and/or document it. Note it is extremely basic but then requires no special casing.

Side effects of this PR:

* The VM generated error logger message which sends a string will nolonger be sent for plug errors
* Should not require many changes once Plug handles exceptions/logging