nginx returns html instead of json response

Hi,

I have configured the below settings in nginx after a few attempts by
tweaking it.

location /apis {
                # First attempt to serve request as file, then
                # as directory, then fall back to displaying a 404.
                fastcgi_intercept_errors off;
                add_header "X-Debug-JSON-APIS" $http_accept always;
                fastcgi_pass 127.0.0.1:9000;
                include fastcgi.conf;
                fastcgi_param SCRIPT_FILENAME $document_root/index.php;
                 }

I have a follow up question related to the below error which appears in
html instead of JSON format when I hit rest api calls
http://mydomain.com/apis in case of when the MySQL Database service is down
as part of testing the end to end flow. The flow is as follows:

User -> Nginx webserver -> PHP-FPM upstream server -> MySQL Database.

The Website Encountered an Unexpected Error. Please try again Later </br>

Is there a way to display the above string in JSON format?

Please guide me. Thanks in advance.

Best Regards,

Kaushal