add handle_syndicate_to_query, fix handle_undelete signature

lib/plug_micropub/handler_behaviour.ex

@@ -30,8 +30,7 @@ defmodule PlugMicropub.HandlerBehaviour do
   @callback handle_undelete(url :: String.t(), access_token) ::
               :ok
               | {:ok, url :: String.t()}
-              | {:error, handler_error}
-              | {:error, handler_error, error_description :: String.t()}
+              | handler_error
 
   @callback handle_config_query(access_token) ::
               {:ok, map}
@@ -41,6 +40,10 @@ defmodule PlugMicropub.HandlerBehaviour do
               {:ok, map}
               | handler_error
 
+  @callback handle_syndicate_to_query(access_token) ::
+              {:ok, map}
+              | handler_error
+
   @callback handle_source_query(
               url :: String.t(),
               properties :: [String.t()],