Fail subsequent posts to Agent after failure

[pitr-ch]

from clojure agent doc: "If any exceptions are thrown by an action function, no nested dispatches will occur, and the exception will be cached in the Agent itself."

[jdantonio]

I'm OK with this change. This Agent was intended to match the behavior of Clojure's agent. It appears that I may have missed the mark.

[jdantonio]

@pitr-ch Was this included in your previous Agent updates or should I leave it open for now. You and the other committers are doing such awesome work I'm starting to lose track! :-)

[pitr-ch]

It was not included. This'll be slightly more complicated. Post calls will have to postponed if Agent.current until end of the update.

[jdantonio]

#412