FIX actually throw error when folding periods/days

lib/Internal/Budget.hs

@@ -1,6 +1,7 @@
 module Internal.Budget (insertBudget) where
 
 import Control.Monad.Except
+import Data.Foldable
 import Database.Persist.Monad
 import Internal.Database.Ops
 import Internal.Types.Main
@@ -235,16 +236,20 @@ workingDays wds start end = fromIntegral $ daysFull + daysTail
     wds' = L.sort $ (\x -> diff (fromWeekday x) startDay) <$> L.nub wds
     diff a b = fromIntegral $ mod (fromEnum a - fromEnum b) 7
 
+-- ASSUME days is a sorted list
 foldDays
   :: MonadInsertError m
   => (Day -> Day -> m a)
   -> Day
   -> [Day]
   -> m [a]
-foldDays f start days
-  -- TODO throw real error here
-  | any (start >) days = throwError undefined
-  | otherwise =
+foldDays f start days = case NE.nonEmpty days of
+  Nothing -> return []
+  Just ds
+    | any (start >) ds ->
+        throwError $
+          InsertException [PeriodError start $ minimum ds]
+    | otherwise ->
         combineErrors $
           snd $
             L.mapAccumL (\prevDay day -> (day, f prevDay day)) start days