fix api_user_get_name

src/webapp/api.v

···
 @['/api/user/get_name'; get]
 fn (mut app App) api_user_get_name(mut ctx Context, username string) veb.Result {
 	if !app.config.instance.public_data {
-		return ctx.server_error('no such error')
+		_ := app.whoami(mut ctx) or {
+			return ctx.unauthorized('no such user')
+		}
 	}
 	user := app.get_user_by_name(username) or { return ctx.server_error('no such user') }
 	return ctx.text(user.get_name())