Return Message ID from SendWarpMessage

[ceyonur]

Currently we don't return anything from SendWarpMessage function as an output. This seems a less weird if it successfully sends a message. We can return the message ID.

Also after #920, we can put this message ID in event for an easier filter for subscribers. Without #920 we won't be able to put this to topics because maximum 3 (+ event ID) topic are supported.