Add additional checks to prevent WsExceptionHandler from throwing undefined errors

[sorenjohanson]

closes #574

Previously, there was no check to see if newNodes actually existed, but since we're returning undefined, newNodes.length may actually throw an error.

[sorenjohanson]

Additional fix: The Promise.reject() in addNodes was never returned properly, so it's propagated directly upwards to the WsExceptionHandler instead of going back to the gateway. It's now returned properly, so it also prevents execution of the reducer.

[JannikStreek]

looking good, thanks!