Find previous URL.

def getPrevUrl(self, url, data):
        """Find previous URL."""
        prevUrl = None
        if self.prevSearch:
            try:
                prevUrl = self.fetchUrl(url, data, self.prevSearch)
            except ValueError as msg:
                # assume there is no previous URL, but print a warning
                out.warn(u"%s Assuming no previous comic strips exist." % msg)
            else:
                prevUrl = self.prevUrlModifier(prevUrl)
                out.debug(u"Found previous URL %s" % prevUrl)
                getHandler().comicPageLink(self.getName(), url, prevUrl)
        return prevUrl