Improve core/css exception messages.

Source/core/css/MediaList.cpp

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2006, 2010, 2012 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 183 matching lines @@
         return queries[index]->cssText();
     return String();
 }
 
 void MediaList::deleteMedium(const String& medium, ExceptionState& exceptionState)
 {
     CSSStyleSheet::RuleMutationScope mutationScope(m_parentRule);
 
     bool success = m_mediaQueries->remove(medium);
     if (!success) {
-        exceptionState.throwUninformativeAndGenericDOMException(NotFoundError);
+        exceptionState.throwDOMException(NotFoundError, "Failed to delete '" + medium + "'.");
         return;
     }
     if (m_parentStyleSheet)
         m_parentStyleSheet->didMutate();
 }
 
 void MediaList::appendMedium(const String& medium, ExceptionState& exceptionState)
 {
     CSSStyleSheet::RuleMutationScope mutationScope(m_parentRule);
 
     bool success = m_mediaQueries->add(medium);
     if (!success) {
-        exceptionState.throwUninformativeAndGenericDOMException(InvalidCharacterError);
+        exceptionState.throwDOMException(InvalidCharacterError, "The value provided ('" + medium + "') is not a valid medium.");
         return;
     }
 
     if (m_parentStyleSheet)
         m_parentStyleSheet->didMutate();
 }
 
 void MediaList::reattach(MediaQuerySet* mediaQueries)
 {
     ASSERT(mediaQueries);
@@ skipping 56 matching lines @@
                     CSSPrimitiveValue* primitiveValue = toCSSPrimitiveValue(cssValue);
                     if (primitiveValue->isDotsPerInch() || primitiveValue->isDotsPerCentimeter())
                         addResolutionWarningMessageToConsole(document, mediaQuerySet->mediaText(), primitiveValue);
                 }
             }
         }
     }
 }
 
 }