Adding error call back function with sendIQ()

this will help to get the error message response by server.
If MAM service is not active in XMPP server, we will get the bellow kind of error message from server:

<iq xmlns="jabber:client" from="[email protected]" to="[email protected]/0F892479AD63271A1558-938994-148287" type="error" xml:lang="en" id="1:sendIQ">
   <query xmlns="urn:xmpp:mam:2">
      <x xmlns="jabber:x:data" type="submit">
         <field var="FORM_TYPE" type="hidden">
            <value>urn:xmpp:mam:2</value>
         </field>
         <field var="with">
            <value>[email protected]</value>
         </field>
         <field var="start">
            <value>2017-05-23T00:00:00Z</value>
         </field>
         <field var="end">
            <value>2017-05-26T00:00:00Z</value>
         </field>
      </x>
      <set xmlns="http://jabber.org/protocol/rsm">
         <max />
         <before />
      </set>
   </query>
   <error code="503" type="cancel">
      <service-unavailable xmlns="urn:ietf:params:xml:ns:xmpp-stanzas" />
   </error>
</iq>

src/strophe.mam.js

@@ -55,6 +55,10 @@ Strophe.addConnectionPlugin('mam', {
         return this._c.sendIQ(iq, function(){
            _c.deleteHandler(handler);
            onComplete.apply(this, arguments);
-        });
+        },
+            function(err){
+                //error callBack function 
+                console.log("Error Response from server:", err);
+            });
     }
 });