Improves error messages returned by the broker in case of a user actionable error happens on node

Replaces the :error message returned by the broker with a user friendly message with Reference ID and also moves the original :error thrown by the node to a :debug message. This will also make sure the broker includes :warning messages (for example app over quota) even when this kind of error happens.

So for example, take an app over quota where you try to add a cartridge. This patch will turn the current response

http://pastebin.com/ap7JB1pv

into something like 

http://pastebin.com/k5A1LXAA