Use better error message for an HTMLMediaElement that has already been connected.

Source/modules/webaudio/AudioContext.cpp

 /*
  * Copyright (C) 2010, Google 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 246 matching lines @@
         exceptionState.throwDOMException(
             InvalidStateError,
             "invalid HTMLMedialElement.");
         return 0;
     }
 
     // First check if this media element already has a source node.
     if (mediaElement->audioSourceNode()) {
         exceptionState.throwDOMException(
             InvalidStateError,
-            "invalid HTMLMediaElement.");
+            "HTMLMediaElement already connected previously to a different MediaElementSourceNode.");
         return 0;
     }
 
     MediaElementAudioSourceNode* node = MediaElementAudioSourceNode::create(this, mediaElement);
 
     mediaElement->setAudioSourceNode(node);
 
     refNode(node); // context keeps reference until node is disconnected
     return node;
 }
@@ skipping 587 matching lines @@
 
     for (HashSet<AudioNode*>::iterator k = m_deferredCountModeChange.begin(); k != m_deferredCountModeChange.end(); ++k)
         (*k)->updateChannelCountMode();
 
     m_deferredCountModeChange.clear();
 }
 
 } // namespace blink
 
 #endif // ENABLE(WEB_AUDIO)