Improve 'performance.{mark,measure} exception messages.

Source/core/page/PerformanceUserTiming.cpp

 /*
  * Copyright (C) 2012 Intel Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 83 matching lines @@
         return;
     }
 
     if (performanceEntryMap.contains(name))
         performanceEntryMap.remove(name);
 }
 
 void UserTiming::mark(const String& markName, ExceptionState& es)
 {
     if (restrictedKeyMap().contains(markName)) {
-        es.throwDOMException(SyntaxError);
+        es.throwDOMException(SyntaxError, "'" + markName + "' is part of the PerformanceTiming interface, and cannot be used as a mark name.");
         return;
     }
 
     double startTime = m_performance->now();
     insertPerformanceEntry(m_marksMap, PerformanceMark::create(markName, startTime));
 }
 
 void UserTiming::clearMarks(const String& markName)
 {
     clearPeformanceEntries(m_marksMap, markName);
 }
 
 double UserTiming::findExistingMarkStartTime(const String& markName, ExceptionState& es)
 {
     if (m_marksMap.contains(markName))
         return m_marksMap.get(markName).last()->startTime();
 
     if (restrictedKeyMap().contains(markName)) {
         double value = static_cast<double>((m_performance->timing()->*(restrictedKeyMap().get(markName)))());
         if (!value) {
-            es.throwDOMException(InvalidAccessError);
+            es.throwDOMException(InvalidAccessError, "'" + markName + "' is empty: either the event hasn't happened yet, or it would provide cross-origin timing information.");
             return 0.0;
         }
         return value - m_performance->timing()->navigationStart();
     }
 
-    es.throwDOMException(SyntaxError);
+    es.throwDOMException(SyntaxError, "The mark '" + markName + "' does not exist.");
     return 0.0;
 }
 
 void UserTiming::measure(const String& measureName, const String& startMark, const String& endMark, ExceptionState& es)
 {
     double startTime = 0.0;
     double endTime = 0.0;
 
     if (startMark.isNull())
         endTime = m_performance->now();
@@ skipping 54 matching lines @@
 {
     return convertToEntrySequence(m_measuresMap);
 }
 
 Vector<RefPtr<PerformanceEntry> > UserTiming::getMeasures(const String& name) const
 {
     return getEntrySequenceByName(m_measuresMap, name);
 }
 
 } // namespace WebCore