Getting a 503 Error before Craft installs - [error] [exception.Craft\HttpException.503] Craft\HttpException

Question

In the browser, I'm getting a page that says:

Service Unavailable
Our site is temporarily unavailable. Please try again later.

In my craft.log, I'm getting a 503 error that says:

******************************************************************************************************
2017/03/14 21:31:04 [error] [exception.Craft\HttpException.503] Craft\HttpException in /var/www/site/craft/app/etc/web/WebApp.php:800
Stack trace:
#0 /var/www/site/craft/app/etc/web/WebApp.php(203): Craft\WebApp->_processInstallRequest()
#1 /var/www/site/craft/app/framework/base/CApplication.php(185): Craft\WebApp->processRequest()
#2 /var/www/site/craft/app/index.php(62): CApplication->run()
#3 /var/www/site/public/index.php(22): require_once('/var/www/site...')
#4 {main}
REQUEST_URI=/
---

******************************************************************************************************

It looks like it's an error when trying to run the installation for Craft. When I visit the server's /admin, I get a 404 page from nginx.

Any and all help is greatly appreciated. Thanks!