No need to worry. Try to think of it this way...nobody serves them clean food in the wild and they've survived thousands of years.
Although not ideal, a little substrate ingestion won't be a problem.
Many people will put a piece of cardboard or paper plate (not paper towel) down before feeding. I used to, be found that the snakes tend to drag the prey off of it anyway.

It's perfectly normal to worry when you see something like that. But your snake will be ok. You don't need to feel bad, it happens all the time.
Good on you for asking though, it shows you care.