Check monad: support for accumulated errors

In addition to warnings, the Check monad in GF.Infra.CheckM can now accumulate
errors. There are two new functions

	checkAccumError: Message -> Check ()
	accumulateError :: (a -> Check a) -> a -> Check a

The former (with the same type as checkWarn) is used to report an accumulated
(nonfatal) error. The latter converts fatal errors into accumulated errors.

Accumulated errors are reported as regular errors by runCheck.

Also, the Check monad type has been made abstract.

src/compiler/GF/Compile/TypeCheck/ConcreteNew.hs

@@ -458,7 +458,7 @@ data MetaValue
 type MetaStore = IntMap.IntMap MetaValue
 data TcResult a
   = TcOk   a MetaStore [Message]
-  | TcFail             [Message]
+  | TcFail             [Message] -- First msg is error, the rest are warnings?
 newtype TcM a = TcM {unTcM :: MetaStore -> [Message] -> TcResult a}
 
 instance Monad TcM where
@@ -480,9 +480,9 @@ tcWarn :: Message -> TcM ()
 tcWarn msg = TcM (\ms msgs -> TcOk () ms ((text "Warning:" <+> msg) : msgs))
 
 runTcM :: TcM a -> Check a
-runTcM f = Check (\ctxt msgs -> case unTcM f IntMap.empty msgs of
-                                  TcOk x _ msgs -> Success x msgs
-                                  TcFail msgs   -> Fail msgs)
+runTcM f = case unTcM f IntMap.empty [] of
+             TcOk x _ msgs -> do checkWarnings msgs; return x
+             TcFail (msg:msgs) -> do checkWarnings msgs; checkError msg
 
 newMeta :: Sigma -> TcM MetaId
 newMeta ty = TcM (\ms msgs ->