Rework error handling in the service protocol and in Observatory.

runtime/vm/profiler_service.cc

Index: runtime/vm/profiler_service.cc
diff --git a/runtime/vm/profiler_service.cc b/runtime/vm/profiler_service.cc
index e073ab54e1b682478de949a5e4f8658cc5c1f0fa..87fe0d04c541b99b5f717be2e74dbf38f1e923ac 100644
--- a/runtime/vm/profiler_service.cc
+++ b/runtime/vm/profiler_service.cc
@@ -2166,9 +2166,7 @@ void ProfilerService::PrintJSON(JSONStream* stream, TagOrder tag_order) {
   MutexLocker profiler_data_lock(isolate->profiler_data_mutex());
   IsolateProfilerData* profiler_data = isolate->profiler_data();
   if (profiler_data == NULL) {
-    JSONObject error(stream);
-    error.AddProperty("type", "Error");
-    error.AddProperty("text", "Isolate does not have profiling enabled.");
+    stream->PrintError(kProfilingDisabled, NULL);
     return;
   }
   SampleBuffer* sample_buffer = profiler_data->sample_buffer();