Show error message on attempt to load old Timeline data into tracing based Timeline

Source/devtools/front_end/timeline/TracingModel.js

Index: Source/devtools/front_end/timeline/TracingModel.js
diff --git a/Source/devtools/front_end/timeline/TracingModel.js b/Source/devtools/front_end/timeline/TracingModel.js
index 3068eec29f9e6c739c3cc752b02dafa4a5213887..85cc897791aae6ce10bb9fcd753d606b27070f7e 100644
--- a/Source/devtools/front_end/timeline/TracingModel.js
+++ b/Source/devtools/front_end/timeline/TracingModel.js
@@ -300,7 +300,10 @@ WebInspector.TracingModel.Loader.prototype = {
 
     finish: function()
     {
-        this._tracingModel._tracingComplete();
+        if (this._sessionIdFound)
+            this._tracingModel._tracingComplete();
+        else
+            WebInspector.console.error(WebInspector.UIString("Trace event %s not found while loading tracing model.", WebInspector.TracingModel.DevToolsMetadataEvent.TracingStartedInPage));
     }
 }