Handle 404s

4c

@@ -20,6 +20,9 @@ try:
 except ValueError:
 	print "That does not look like a valid URL."
 	exit(1)
+except urllib2.HTTPError:
+	print "The given URL returns a HTTP 404 status code - the thread may have died."
+	exit(1)
 
 if options['newdir'] == True:
 	thread_id = url.split('/')[-1]