Conditional Nginx error pages based on Accept (or other) headers

nginx.conf

http {
  map $http_accept $extension {
    default html;
    application/json json;
  }

  server {
    listen 80;

    error_page 500 /500;

    location = /500 {
      internal;
      alias /var/www/error/403.$extension;
    }
  }
}

map $content_type $extension {
  default html;
  ~application/json json;
}