plugin XEP-0060: raise an NotFound exception instead of StanzaError in subscribe:

`exceptions.NotFound` is now raised in stead of `item-not-found` `StanzaError` for
consistency with `getItems` and because it's easier to catch (condition has to be checked
when `StanzaError` is received).

rel 372

sat/plugins/plugin_xep_0060.py

@@ -1207,10 +1207,16 @@ class XEP_0060(object):
         )
         if not cont:
             return trigger_sub
-        subscription = await client.pubsub_client.subscribe(
-            service, nodeIdentifier, sub_jid or client.jid.userhostJID(), options=options,
-            sender=client.jid.userhostJID()
-        )
+        try:
+            subscription = await client.pubsub_client.subscribe(
+                service, nodeIdentifier, sub_jid or client.jid.userhostJID(),
+                options=options, sender=client.jid.userhostJID()
+            )
+        except error.StanzaError as e:
+            if e.condition == 'item-not-found':
+                raise exceptions.NotFound(e.text or e.condition)
+            else:
+                raise e
         return subscription
 
     def _unsubscribe(self, service, nodeIdentifier, profile_key=C.PROF_KEY_NONE):