wesnoth_addon_manager: exit gracefully when removing a non-existing add-on

data/tools/wesnoth/campaignserver_client.py

@@ -303,6 +303,8 @@ class CampaignClient:
         return data2
 
     def decode_WML(self, data):
+        if data is None:
+            return None
         p = wmlparser.Parser()
         p.parse_binary(data)
         return p.root
@@ -341,6 +343,8 @@ class CampaignClient:
             passphrase = passphrase)
 
         self.send_packet(self.make_packet(request))
+
+        # if a missing add-on is specified read_packet returns None
         return self.decode(self.read_packet())
 
     def change_passphrase(self, name, old, new):

data/tools/wesnoth_addon_manager

@@ -303,7 +303,10 @@ if __name__ == "__main__":
     elif args.remove:
         cs = CampaignClient(address, secure=args.secure, ipv6=args.ipv6)
         data = cs.delete_campaign(*args.remove)
-        print_messages(data)
+        if data is None:
+            print("Failed to remove add-on '{}'.".format(args.remove[0]), file=sys.stderr)
+        else:
+            print_messages(data)
 
     elif args.change_passphrase:
         cs = CampaignClient(address, secure=args.secure, ipv6=args.ipv6)