Log the err and call with null, then return.

src/converse-core.js

@@ -142,7 +142,9 @@
             errback = callback;
         }
         if (!converse.features.findWhere({'var': Strophe.NS.MAM})) {
-            errback('This server does not support XEP-0313, Message Archive Management');
+            converse.log('This server does not support XEP-0313, Message Archive Management');
+            errback(null);
+            return;
         }
         var queryid = converse.connection.getUniqueId();
         var attrs = {'type':'set'};