web: fix error message being a bit too persistent

visiting ?add=1 afresh was still showing the previous visit's error

hledger-web/Foundation.hs

@@ -105,6 +105,7 @@ instance Yesod App where
 
     defaultLayout widget = do
         master <- getYesod
+        lastmsg <- getMessage
         vd@VD{..} <- getViewData
 
         -- We break up the default layout into two components:

hledger-web/templates/default-layout-wrapper.hamlet

@@ -100,7 +100,7 @@ $newline never
                 <button type="button" .close data-dismiss="modal" aria-hidden="true">&times;
                 <h3 .modal-title #addLabel>Add a transaction
               <div .modal-body>
-                $maybe m <- msg
+                $maybe m <- lastmsg
                   $if isPrefixOf "Errors" (renderHtml m)
                     <div #message>#{m}
                 ^{addform staticRootUrl vd}

hledger-web/templates/default-layout.hamlet

@@ -1,4 +1,4 @@
-$maybe m <- msg
+$maybe m <- lastmsg
   $if not $ isPrefixOf "Errors" (renderHtml m)
     <div #message>#{m}
 ^{widget}