Cleanup VM service

runtime/bin/vmservice/running_isolates.dart

Index: runtime/bin/vmservice/running_isolates.dart
diff --git a/runtime/bin/vmservice/running_isolates.dart b/runtime/bin/vmservice/running_isolates.dart
index 0cfa02f3048542f47d300a39124075b14c240378..547b9b5e22e3cd3f81b3152f916abcbf21ed1ff5 100644
--- a/runtime/bin/vmservice/running_isolates.dart
+++ b/runtime/bin/vmservice/running_isolates.dart
@@ -40,10 +40,12 @@ class RunningIsolates implements ServiceRequestRouter {
 
   Future route(ServiceRequest request) {
     if (request.pathSegments.length == 0) {
-      return null;
+      request.setErrorResponse('No path.');
+      return new Future.value(request);
     }
     if (request.pathSegments[0] != 'isolates') {
-      return null;
+      request.setErrorResponse('Path must begin with /isolates/.');
+      return new Future.value(request);
     }
     if (request.pathSegments.length == 1) {
       // Requesting list of running isolates.