guard against any remaining infinite recursion bugs; throw error if no candidate cn be found in next hundred years

Note that the exception thrown is not visible in the webapp currently
because it crashes one of Cronner's 2 worker threads, which is never
checked.

Utility/Scheduled.hs

@@ -86,7 +86,7 @@ nextTime schedule lasttime = do
 {- Calculate the next time that fits a Schedule, based on the
  - last time it occurred, and the current time. -}
 calcNextTime :: Schedule -> Maybe LocalTime -> LocalTime -> Maybe NextTime
-calcNextTime (Schedule recurrance scheduledtime) lasttime currenttime
+calcNextTime schedule@(Schedule recurrance scheduledtime) lasttime currenttime
 	| scheduledtime == AnyTime = do
 		next <- findfromtoday True
 		return $ case next of
@@ -108,7 +108,13 @@ calcNextTime (Schedule recurrance scheduledtime) lasttime currenttime
 	window startd endd = NextTimeWindow
 		(LocalTime startd nexttime)
 		(LocalTime endd (TimeOfDay 23 59 0))
-	findfrom r afterday candidate = case r of
+	findfrom r afterday candidate
+		| ynum candidate > (ynum (localDay currenttime)) + 100 =
+			-- avoid possible infinite recusion
+			error $ "bug: calcNextTime did not find a time within 100 years to run " ++
+			show (schedule, lasttime, currenttime)
+		| otherwise = findfromChecked r afterday candidate
+	findfromChecked r afterday candidate = case r of
 		Daily
 			| afterday -> Just $ exactly $ addDays 1 candidate
 			| otherwise -> Just $ exactly candidate