@jason.s.blake, as @Nocodify said you can’t customize these popups but you can add error handling to eliminate them.
In your workflows, find the ‘An element has an error running a workflow’ event
In this case you would choose the Login Button which would give you an event like this
Now you can do whatever you want with the error. Create your own popup, display a toast notification, etc.
Just add actions to this workflow as normal. In this case, I’m simply displaying the same error message but in a popup of my own design
Hope it helps!