Added default overwrite in error_page 494.

Added default overwrite in error_page 494.

We used to have default error_page overwrite for 495, 496, and 497, so
a configuration like

    error_page 495 /error;

will result in error 400, much like without any error_page configured.

The 494 status code was introduced later (in 3848:de59ad6bf557, nginx 0.9.4),
and relevant changes to ngx_http_core_error_page() were missed, resulting
in inconsistent behaviour of "error_page 494" - with error_page configured
it results in 494 being returned instead of 400.

Reported by Frank Liu,
http://mailman.nginx.org/pipermail/nginx/2020-February/058957.html.