xep_0384: do things in session_bind only if lib is available

`session_bind` is apparently loaded at about the same time as
plugin_init, and exceptions will be raise if the library could not be
loaded.

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

plugin.py

@@ -144,7 +144,7 @@ class XEP_0384(BasePlugin):
         if not self.backend_loaded:
             log.info("xep_0384 cannot be loaded as the backend omemo library "
                      "is not available")
-            return None
+            raise PluginCouldNotLoad
 
         if not self.data_dir:
             log.info("xep_0384 canoot be loaded as there is not data directory "
@@ -183,6 +183,7 @@ class XEP_0384(BasePlugin):
         self.xmpp['xep_0163'].remove_interest(OMEMO_DEVICES_NS)
 
     def session_bind(self, _jid):
+        if self.backend_loaded:
             self.xmpp['xep_0163'].add_interest(OMEMO_DEVICES_NS)
             asyncio.ensure_future(self._set_device_list())
             asyncio.ensure_future(self._publish_bundle())