Don't return null when being unable to deserialize a DropMessage's data field

Hopefully it won't break stuff and/or destroys the world 
- one less possibility of having NREs though!
I justified throwing a 'wrapping' JsonParseException by only having to handle exactly one exception type in outer method caller scopes. 
Furthermore, the ClassNotFoundException is handed through, so it still should be acceptable.
I may alter the method's signature otherwise, if somebody thinks that this would be better.