Feature to return placeholder images added to error page handler

If the feature is enabled in the OSGI config of the ErrorPageHandlerImpl (is disabled by default) one can specify a path to a placeholder image which is returned in the case an error (404, 500, ...) occurs and the current request is an image request. The path is relative to the found error page thus you can use different images for different sub trees of the content. An image request is identified by the extension of the request. The list of extensions to handle is configurable. The image can be any type of image the RequestDispatcher can handle (file in the JCR, DAM asset, image component).