[#86933] Validate quantity values

This should avoid the `nil can't be coerced into BigDecimal` exceptions we were seeing.

It looks like the typical case was the Ajax event firing in the middle of the user making a change to the quantity field. The quantity field would be blank, and the back end would 500. There are two changes here: on the front end, to fire only when the quantity value contains a number, and on the back end, to error out more gracefully when sent a non-numeric quantity.