Add error message if muc service not found

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

poezio/core/core.py

@@ -882,6 +882,12 @@ class Core:
         # not, give up
         default_muc = config.get('default_muc_service')
         if not default_muc:
+            self.information(
+                "Error finding a MUC service to join. If your server does not "
+                "provide one, set 'default_muc_service' manually to a MUC "
+                "service that allows room creation.",
+                'Error'
+            )
             return
 
         nick = self.own_nick