RemoteStep#processNextStart() throws CompletionException instead of underlying exception

There is a slight behavior change in TinkerPop 3.2.4 as it pertains to an exception thrown when processing a traversal on a remote traversal source.

In 3.2.3, if an unchecked exception is thrown, it is propagated directly to the user.

However, in 3.2.4, since RemoteStep#processNextStart() now calls RemoteStep#promise() and joins on the returning futures result, if the future fails a CompletionException is thrown wrapping the cause, instead of the cause itself.

For consistency, if that exception is an unchecked exception, it should be thrown directly, otherwise it should be wrapped in an IllegalStateException as it did before.