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

Source/devtools/front_end/timeline/TracingModel.js

 /*
  * Copyright 2014 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 /**
  * @constructor
  * @extends {WebInspector.SDKObject}
  */
@@ skipping 282 matching lines @@
         }
 
         if (this._sessionIdFound) {
             this._tracingModel._tracingStarted(sessionId);
             this._tracingModel._eventsCollected(this._events);
         }
     },
 
     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));
     }
 }
 
 
 /**
  * @constructor
  * @param {!WebInspector.TracingModel.EventPayload} payload
  * @param {number} level
  * @param {?WebInspector.TracingModel.Thread} thread
  */
@@ skipping 345 matching lines @@
     started: function(consoleTimeline, sessionId)
     {
         this._tracingModel._tracingStarted(sessionId);
     },
 
     stopped: function()
     {
         this._tracingModel._onStop();
     }
 }