Make periodic txns generation not silently fail when end date is older than start date

[v-Nyo]

I spent an embarrassing amount of time debugging my scripts because no txns where generated by this:

~ monthly from 2025-08-10 to 2025-01-10

Would be nice if it could fail loudly if the second date is older than start date. That said no idea if implementing a check for this minor thing is a good idea.