Improve modules' TypeError exception messages.

Source/modules/encoding/TextEncoder.cpp

Index: Source/modules/encoding/TextEncoder.cpp
diff --git a/Source/modules/encoding/TextEncoder.cpp b/Source/modules/encoding/TextEncoder.cpp
index 98b935497dc8cdfac456095abfc0044f015937e6..2ede043797c32167a94e199561d264b373e20b94 100644
--- a/Source/modules/encoding/TextEncoder.cpp
+++ b/Source/modules/encoding/TextEncoder.cpp
@@ -45,13 +45,13 @@ PassRefPtr<TextEncoder> TextEncoder::create(const String& utfLabel, ExceptionSta
 
     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;
     }