gob register AppError (#9182)

This allows plugin hooks to return the result of calling the API as an
`error`.

plugin/client_rpc.go

@@ -67,6 +67,7 @@ func init() {
 	gob.Register([]*model.SlackAttachment{})
 	gob.Register([]interface{}{})
 	gob.Register(map[string]interface{}{})
+	gob.Register(&model.AppError{})
 }
 
 // These enforce compile time checks to make sure types implement the interface