Update exceptions to support bodies

[dylanneild]

Update exceptions to support bodies; right now when an exception is thrown that generates an HTTP error message, the server simply returns an HTTP 1.1 error to the client without a body.

Add a simply body to these messages that allows for more specific error condition messages to be returned back to the client. This should also be selectable when the exception is thrown so people implementing clients can choose just to send blank errors if they want to.