better invalid date error message

This is more likely to occur when getting the constraint columns in the wrong
order than any other kind of user or programming error, so a vanilla error
message is more useful than a stack trace here.

vice-speaker-rotator.py

@@ -13,7 +13,11 @@ https://cs.uwaterloo.ca/twiki/view/CrySP/UpcomingEvents
 """
 
 def str_to_date(string_date):
-    return date(*[int(s) for s in string_date.split('-')])
+    try:
+        return date(*[int(s) for s in string_date.split('-')])
+    except:
+        print("invalid date: " + string_date)
+        sys.exit()
 
 class CryspCalendar:
     """Representation of a CrySP weekly meeting schedule for a term"""