Improve modules' TypeError exception messages.

Source/modules/indexeddb/IDBTransaction.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
@@ skipping 271 matching lines @@
 }
 
 IndexedDB::TransactionMode IDBTransaction::stringToMode(const String& modeString, ExceptionState& exceptionState)
 {
     if (modeString.isNull()
         || modeString == IDBTransaction::modeReadOnly())
         return IndexedDB::TransactionReadOnly;
     if (modeString == IDBTransaction::modeReadWrite())
         return IndexedDB::TransactionReadWrite;
 
-    exceptionState.throwUninformativeAndGenericTypeError();
+    exceptionState.throwTypeError("The mode provided ('" + modeString + "') is not one of 'readonly' or 'readwrite'.");
     return IndexedDB::TransactionReadOnly;
 }
 
 const AtomicString& IDBTransaction::modeToString(IndexedDB::TransactionMode mode)
 {
     switch (mode) {
     case IndexedDB::TransactionReadOnly:
         return IDBTransaction::modeReadOnly();
         break;
 
@@ skipping 74 matching lines @@
     event->setTarget(this);
     eventQueue->enqueueEvent(event);
 }
 
 blink::WebIDBDatabase* IDBTransaction::backendDB() const
 {
     return m_database->backend();
 }
 
 } // namespace WebCore