Add permission error when user cannot write to /etc/nginx folder

[poespas]

Currently users get a very vague message when they run the nginx_config_reloader on an user that isn't able to write to /etc/nginx, this PR should solve this and add the check.

Before:

2024-12-30 06:25:19,553 nginx_config_reloader INFO     /data/web/nginx
2024-12-30 06:25:19,553 nginx_config_reloader DEBUG    Applying new config
2024-12-30 06:25:19,647 nginx_config_reloader ERROR    Installation of magento config failed

After:

2024-12-30 06:22:14,216 nginx_config_reloader INFO     /data/web/nginx
2024-12-30 06:22:14,216 nginx_config_reloader DEBUG    Applying new config
2024-12-30 06:22:14,229 nginx_config_reloader ERROR    No write permissions to main nginx config directory, please check your permissions.

[tdgroot]

LGTM, can you add a test to reflect the changes in _apply?