make status returning more consistent

Function `_M.send_404()` and `_M.send_500()` are returning `status` which meant to be HTTP Status Code, in turn `_M.run()` returning result of `_M.send_404()` or `_M.send_500()` which also has meaning of HTTP Status Code. In common loops like cgi.lua and fastcgi.lua these returnings are not used, but if they would consistency would be violated.