Fix Member model method last_paid

The last_paid method used DoesNotExist exception to handle situation
where no proper payment models were found. However, the syntax was
wrong and it should have used Payment.DoesNotExist. This was fixed.

members/models.py

@@ -103,7 +103,7 @@ class Member(BaseMember):
             latest = payments.latest('date')
             date = latest.date
             return date
-        except DoesNotExist:
+        except Payment.DoesNotExist:
             return None
 
     @staticmethod