Investigate validation/coercion of SSE / Websockets

[oliyh]

Could be an interesting experiment

[oliyh]

Validation errors need to return something sensible. Detect by putting the exception on the channel with normal ex-data.

Regarding serialisation, Pedestal currently does pr-str on events so interestingly enough, edn "just works", but JSON or anything else would require normal serialisation. Response type probably needs to be text/event-stream to avoid breaking other things but could try something like text/event-stream+json along the lines of transit. Content negotiation would need to be aware of this.