Currently this can't be done unless you send emails to all the users with the help of PowerShell. However, in this case you will have to put all the logic of the Scheduled Task in the PowerShell script, and also there can't be a global solution for all the Scheduled Tasks. You'll have to deal with each task one-by-one. This seems completely counter-productive.
Instead, we would suggest making some sort of a test run to make sure that a Scheduled Task works correctly. For example, if you are not sure about the conditions in your Scheduled Task, on the 1st run, instead of sending emails, you can modify a certain property of the AD objects that you don't use. It is a simple action that will not raise unnecessary attention and that you can easily undo. When completely satisfied with the test results, you can replace the action that modifies the property with the Send e-mail notification action.