[Gate] [ActionsReport] Publish error and return 503 when a task fails

This change adds to the Action object an 'error' property (class ActionErrror)
to provide basic information about failure.

When the task fails, a task can now intercept exception and publish it with
$actionToReport->attachExcpeion($ex). Type and message should be checked
and asserted not to expose sensible information. A custom exception would
probably more safe in the last case.

The response code will be 503 instead of 200. If ActionsReport feature is
disabled, we don't inform the client of how we handle the payload, and so
a 200 is still returned.

Here a sample of reply when the message broker is down:

{
  "actions": [
    {
      "method": "publish",
      "target": "github_events",
      "routingKey": "nasqueron.orgz.ping",
      "action": "Nasqueron\Notifications\Actions\AMQPAction",
      "error": {
        "type": "PhpAmqpLib\Exception\AMQPRuntimeException",
        "message": "Error Connecting to server (101): Network is unreachable"
      }
    }
  ],
  "created": 1449468356,
  "gate": "GitHub",
  "door": "Nasqueron"
}