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

Source/modules/webaudio/AudioContext.cpp

Index: Source/modules/webaudio/AudioContext.cpp
diff --git a/Source/modules/webaudio/AudioContext.cpp b/Source/modules/webaudio/AudioContext.cpp
index 39f6180ad733c492b35a46d1548be2d553d54d1e..e8b2c9cb721f98a30f28d4469e7b128eee224a4f 100644
--- a/Source/modules/webaudio/AudioContext.cpp
+++ b/Source/modules/webaudio/AudioContext.cpp
@@ -264,7 +264,7 @@ MediaElementAudioSourceNode* AudioContext::createMediaElementSource(HTMLMediaEle
     if (mediaElement->audioSourceNode()) {
         exceptionState.throwDOMException(
             InvalidStateError,
-            "invalid HTMLMediaElement.");
+            "HTMLMediaElement already connected previously to a different MediaElementSourceNode.");
         return 0;
     }