Error on recurring payment with one installment
To make it even more confusing -> installments = 0 actually means unlimited/open-ended recurring contributions in CiviCRM. 

Your options:


Do not expose the installments field (that will then be = 0 or NULL
by default > so open-ended). 
Add some jQuery to check the value of that field give the user a
notice that it can not be = 1. How to add jQuery - there are a number
of ways to do this - depending on the CMS that you use.