Improve modules' TypeError exception messages.

Source/modules/encoding/TextEncoder.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 27 matching lines @@
 #include "wtf/text/TextEncodingRegistry.h"
 
 namespace WebCore {
 
 PassRefPtr<TextEncoder> TextEncoder::create(const String& utfLabel, ExceptionState& exceptionState)
 {
     const String& encodingLabel = utfLabel.isNull() ? String("utf-8") : utfLabel;
 
     WTF::TextEncoding encoding(encodingLabel);
     if (!encoding.isValid()) {
-        exceptionState.throwUninformativeAndGenericTypeError();
+        exceptionState.throwTypeError("The encoding label provided ('" + encodingLabel + "') is invalid.");
         return 0;
     }
 
     String name(encoding.name());
     if (name != "UTF-8" && name != "UTF-16LE" && name != "UTF-16BE") {
-        exceptionState.throwUninformativeAndGenericTypeError();
+        exceptionState.throwTypeError("The encoding label provided ('" + encodingLabel + "') is not one of 'UTF-8', 'UTF-16LE', or 'UTF-16BE'.");

[jsbell, 2013/12/20 17:47:50]

Nit: Lots of labels aliases for these are accepted, and these aren't the
preferred names per the spec.

Suggest: 'utf-8', 'utf-16', or 'utf-16be'

[jsbell, 2013/12/20 17:51:08]

And maybe "encoding provided" instead of "encoding label provided"; the previous
isValid() test rules out invalid *labels*; at this point the label is valid,
it's the encoding that's forbidden.

         return 0;
     }
 
     return adoptRef(new TextEncoder(encoding.name()));
 }
 
 TextEncoder::TextEncoder(const String& encoding)
     : m_encoding(encoding)
     , m_codec(newTextCodec(m_encoding))
 {
@@ skipping 24 matching lines @@
     else
         result = m_codec->encode(input.characters16(), input.length(), WTF::QuestionMarksForUnencodables);
 
     const char* buffer = result.data();
     const unsigned char* unsignedBuffer = reinterpret_cast<const unsigned char*>(buffer);
 
     return Uint8Array::create(unsignedBuffer, result.length());
 }
 
 } // namespace WebCore