Wordpress update error

Hi, today i got an error when i went to update my wordpress on to the latest version. its a sub-directory multi-site setup by the way.

when i click on update, after some time i get the following error on browser

mysite.com is currently unable to handle this request.
HTTP ERROR 500

and this is what i got from error log

user@mysite:~$ sudo tail -50 /var/log/nginx/error.log
2017/03/28 09:52:34 [error] 3559#3559: *2 FastCGI sent in stderr: "PHP message: PHP Fatal error: Uncaught Error: Call to undefined function get_user_locale() in /var/www/html/wp-admin/about.php:22
Stack trace:
#0 /var/www/html/wp-admin/network/about.php(13): require()
#1 {main}
thrown in /var/www/html/wp-admin/about.php on line 22" while reading response header from upstream, client: 192.205.134.114, server: mysite.com, request: "GET /wp-admin/network/about.php?updated HTTP/1.1", upstream: "fastcgi://unix:/run/php/php7.0-fpm.sock:", host: "mysite.com", referrer: "http://mysite.com/wp-admin/network/update-core.php?action=do-core-upgrade"

but when i again access to wp-admin i found the wordpress is updated. but i am worried that i got a broken update and that might occur issues in future. anyone got any idea whats could be the reason?

this is what i got inside location php by the way

        location ~ \.php$ {
                fastcgi_split_path_info ^(.+\.php)(/.+)$;
                include snippets/fastcgi-php.conf;
                fastcgi_pass unix:/run/php/php7.0-fpm.sock;
                include fastcgi_params;
                fastcgi_param REQUEST_METHOD $request_method;
                fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
                fastcgi_cache MYAPP;
                fastcgi_cache_valid 200 60m;
                fastcgi_cache_bypass $no_cache;
                fastcgi_no_cache $no_cache;
        }

thanks in advance.