Send errors back to the client

[thilinamb]

Example: when queried for a non-existing dataset

sustain.synopsis.dht.store.query.QueryException: Non existing dataset: noaa_2015_jan
    at sustain.synopsis.dht.store.node.NodeStore.getMatchingEntityStores(NodeStore.java:285)
    at sustain.synopsis.dht.store.query.QueryCoordinator.schedule(QueryCoordinator.java:29)
    at sustain.synopsis.dht.services.TargetedQueryService.query(TargetedQueryService.java:25)
    at sustain.synopsis.dht.store.services.TargetedQueryServiceGrpc$MethodHandlers.invoke(TargetedQueryServiceGrpc.java:210)
    at io.grpc.stub.ServerCalls$UnaryServerCallHandler$UnaryServerCallListener.onHalfClose(ServerCalls.java:172)
    at io.grpc.internal.ServerCallImpl$ServerStreamListenerImpl.halfClosed(ServerCallImpl.java:331)
    at io.grpc.internal.ServerImpl$JumpToApplicationThreadServerStreamListener$1HalfClosed.runInContext(ServerImpl.java:817)
    at io.grpc.internal.ContextRunnable.run(ContextRunnable.java:37)
    at io.grpc.internal.SerializingExecutor.run(SerializingExecutor.java:123)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)