fix to add an custom callback if wrong url
fix to add an custom callback if wrong url

if the url do not link to RSS Feed, responseData is null and throws
an error while reading the property feed
the option callbackWrongURL makes the use of custom error handling
possible
superfeedr/jquery-rss,Aurif3x/jquery-rss,landsurveyorsunited/jquery-rss,Aurif3x/jquery-rss,rsallo/jquery-rss,landsurveyorsunited/jquery-rss,sdepold/jquery-rss,sdepold/jquery-rss,superfeedr/jquery-rss,rsallo/jquery-rss