Introduce SassValueWrapper::fail()

This method uses Sass error value
as a vehicle to report the error message,
but returns NULL to indicate constructor
failure.

This is used to tell successful creation
of the Sass error object from the constructor
failure, where no valid Sass object can be
created.

src/sass_types/sass_value_wrapper.h

@@ -25,6 +25,7 @@ namespace SassTypes
       static v8::Local<v8::Function> get_constructor();
       static v8::Local<v8::FunctionTemplate> get_constructor_template();
       static NAN_METHOD(New);
+      static Sass_Value *fail(const char *, Sass_Value **);
 
     protected:
       Sass_Value* value;
@@ -119,6 +120,12 @@ namespace SassTypes
   T* SassValueWrapper<T>::unwrap(v8::Local<v8::Object> obj) {
     return static_cast<T*>(Factory::unwrap(obj));
   }
+
+  template <class T>
+  Sass_Value *SassValueWrapper<T>::fail(const char *reason, Sass_Value **out) {
+    *out = sass_make_error(reason);
+    return NULL;
+  }
 }