ui: clarify and refine journal reloading a bit

The CLI options saved in the UI state are not updated if reloading fails.
(I didn't need this change after all, but it seems reasonable.)

Reloading on the error screen just updates the message, rather than
entering a new error screen.

Docs have been clarified.

hledger-ui/Hledger/UI/ErrorScreen.hs

@@ -105,12 +105,34 @@ hledgerparseerrorpositionp = do
   c <- read <$> many1 digit
   return (f, l, c)
 
--- If journal file(s) have changed, reload the journal and regenerate all screens.
--- This is here so it can reference the error screen.
+-- Unconditionally reload the journal, regenerating the current screen
+-- and all previous screens in the history.
+-- If reloading fails, enter the error screen, or if we're already
+-- on the error screen, update the error displayed.
+-- The provided CliOpts are used for reloading, and then saved
+-- in the UIState if reloading is successful (otherwise the
+-- ui state keeps its old cli opts.)
+-- Defined here so it can reference the error screen.
+uiReloadJournal :: CliOpts -> Day -> UIState -> IO UIState
+uiReloadJournal copts d ui = do
+  ej <- journalReload copts
+  return $ case ej of
+    Right j  -> regenerateScreens j d ui{aopts=(aopts ui){cliopts_=copts}}
+    Left err ->
+      case ui of
+        UIState{aScreen=s@ErrorScreen{}} -> ui{aScreen=s{esError=err}}
+        _                                -> screenEnter d errorScreen{esError=err} ui
+
+-- Like uiReloadJournal, but does not bother re-parsing the journal if
+-- the file(s) have not changed since last loaded. Always regenerates
+-- the current and previous screens though, since opts or date may have changed.
 uiReloadJournalIfChanged :: CliOpts -> Day -> Journal -> UIState -> IO UIState
 uiReloadJournalIfChanged copts d j ui = do
-  (ej, _) <- journalReloadIfChanged copts d j
+  (ej, _changed) <- journalReloadIfChanged copts d j
   return $ case ej of
-    Right j' -> regenerateScreens j' d ui
-    Left err -> screenEnter d errorScreen{esError=err} ui
+    Right j' -> regenerateScreens j' d ui{aopts=(aopts ui){cliopts_=copts}}
+    Left err ->
+      case ui of
+        UIState{aScreen=s@ErrorScreen{}} -> ui{aScreen=s{esError=err}}
+        _                                -> screenEnter d errorScreen{esError=err} ui