Improve modules' TypeError exception messages.

Source/modules/encoding/TextDecoder.cpp

 /*
  * Copyright (C) 2013 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 26 matching lines @@
 #include "wtf/text/TextEncodingRegistry.h"
 
 namespace WebCore {
 
 PassRefPtr<TextDecoder> TextDecoder::create(const String& label, const Dictionary& options, ExceptionState& exceptionState)
 {
     const String& encodingLabel = label.isNull() ? String("utf-8") : label;
 
     WTF::TextEncoding encoding(encodingLabel);
     if (!encoding.isValid()) {
-        exceptionState.throwUninformativeAndGenericTypeError();
+        exceptionState.throwTypeError("The encoding label provided ('" + encodingLabel + "') is invalid.");
         return 0;
     }
 
     bool fatal = false;
     options.get("fatal", fatal);
 
     return adoptRef(new TextDecoder(encoding.name(), fatal));
 }
 
 
@@ skipping 47 matching lines @@
             s.remove(0);
     }
 
     if (flush)
         m_bomSeen = false;
 
     return s;
 }
 
 } // namespace WebCore