Fail SSIS "Execute Process Task" if false is returned
Try using a Script Task to check the value of the SSIS variable that is set as the StandardOutputVariable of the Execute Process Task.  If this value is false the Dts.Events.FireError method raises an error event that can be used to fail the package.  The code below is an example of this using C#, where an error is raised when the variable is false.  The SSIS variable will need to be set in the ReadOnlyVariables field of the Script Task to allow it to be accessed.

    if (!Convert.ToBoolean(Dts.Variables["User::YourVariable"].Value))
    {
        Dts.Events.FireError(0, "Error", "Error Raised from Folder Check", null, 0);
    }