connected_hook: allow hook to specify an error message.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/plugins/reject.py

@@ -16,7 +16,7 @@ plugin = Plugin()
 def on_connected(peer, plugin):
     if peer['id'] in plugin.reject_ids:
         print("{} is in reject list, disconnecting".format(peer['id']))
-        return {'result': 'disconnect'}
+        return {'result': 'disconnect', 'error_message': 'You are in reject list'}
 
     print("{} is allowed".format(peer['id']))
     return {'result': 'continue'}