Improve core/css exception messages.

Source/core/css/FontFace.cpp

Index: Source/core/css/FontFace.cpp
diff --git a/Source/core/css/FontFace.cpp b/Source/core/css/FontFace.cpp
index aac3b7e4adda019bfc6f6e90b0456158b606e72f..f54d23258f1ca351183c55b6dd1f1e433786c481 100644
--- a/Source/core/css/FontFace.cpp
+++ b/Source/core/css/FontFace.cpp
@@ -102,7 +102,7 @@ PassRefPtr<FontFace> FontFace::create(const AtomicString& family, const String&
 {
     RefPtr<CSSValue> src = parseCSSValue(source, CSSPropertySrc);
     if (!src || !src->isValueList()) {
-        exceptionState.throwUninformativeAndGenericDOMException(SyntaxError);
+        exceptionState.throwDOMException(SyntaxError, "The source provided ('" + source + "') could not be parsed as a value list.");
         return 0;
     }
 
@@ -246,7 +246,7 @@ void FontFace::setPropertyFromString(const String& s, CSSPropertyID propertyID,
 {
     RefPtr<CSSValue> value = parseCSSValue(s, propertyID);
     if (!value || !setPropertyValue(value, propertyID))
-        exceptionState.throwUninformativeAndGenericDOMException(SyntaxError);
+        exceptionState.throwDOMException(SyntaxError, "Failed to set '" + s + "' as a property value.");
 }
 
 bool FontFace::setPropertyFromStyle(const StylePropertySet* properties, CSSPropertyID propertyID)