CS-14687: When a valid instance id that does not belong to the current user is passed to ec2-stop-instances,ec2-start-instances,ec2-terminate-instances, api does not return anythin

Changes:
Throw error if LookUpInstances fails to list the Vm by id.

awsapi/src/com/cloud/bridge/service/core/ec2/EC2Engine.java

@@ -1795,6 +1795,11 @@ public class EC2Engine {
     			}
     			instances.addInstance(ec2Vm);
     		}
+		}else{
+		    if(instanceId != null){
+		        //no such instance found
+		        throw new EC2ServiceException(ServerError.InternalError, "Instance:" + instanceId + " not found");
+		    }
 		}
 		return instances;
 	}