Despite the disappointing loss, a late surge from the Saints saw them earn a bonus point across the Channel.
"We're not out of it," Hartley told Sky Sports News. "We've got Ulster next at our place, and we'll know where we are in terms of qualification a lot better after that one. There is still a long way to go in the competition.
"We knew that if we were to be considered contenders for the competition, we need to win games at home against the likes of Ulster and that's where our focus now turns to."
Friday night's other match in Pool 4 saw Ulster keep up their unbeaten start to the competition with a 19-6 victory at Glasgow Warriors.