Use _doing_it_wrong in initialization check

[coreymckrill]

Replaces the error_log call with WP Core's _doing_it_wrong function. The latter uses trigger_error, which is more versatile than error_log because its behavior can be customized with an error handler. _doing_it_wrong also only triggers an error if WP_DEBUG is set to true, so it won't add noise to logs in production environments.

To test

1. In your testing environment, set WP_DEBUG to true. Also, either install and activate Query Monitor (for viewing PHP errors) or set WP_DEBUG_LOG to true.
2. Add the following snippet to a file in your mu-plugins directory:

   add_action(
   	'plugins_loaded',
   	function() {
   		as_schedule_single_action( time() + HOUR_IN_SECONDS, 'not_a_real_hook' );
   	}
   );
   

3. In WP Admin, navigate to Tools > Scheduled Actions. There should not be a scheduled event for not_a_real_hook, because as_schedule_single_action was called before Action Scheduler was fully initialized.
4. Now check your PHP errors, either in the Query Monitor console or in your error log file. You should see the following error:

   Function as_schedule_single_action was called incorrectly. as_schedule_single_action() was called before the Action Scheduler data store was initialized Please see Debugging in WordPress for more information. (This message was added in version 3.1.6.)

5. Now set WP_DEBUG to false and refresh the Scheduled Actions screen. You should still not see a scheduled event for not_a_real_hook. Additionally, though, there should not be a PHP error message this time, because debug has been disabled.

[barryhughes]

Looks good!

• Tests as per description.
• Seems like a better way to surface this problem.
• Was initially a little unsure if 3.1.6 was appropriate as the introduced version number, but on reflection I think it makes sense.

Thanks, @coreymckrill!