Handle over-applied custom type errors too.

Consider

    type family F :: Type -> Type where
       F = TypeError (Text "Error")

Now, if we see something like `F Int` we should still report the custom
type error.

compiler/typecheck/TcRnTypes.hs

@@ -1733,9 +1733,9 @@ isTypeHoleCt (CHoleCan { cc_hole = TypeHole }) = True
 isTypeHoleCt _ = False
 
 -- | The following constraints are considered to be a custom type error:
---    1. TypeError msg
---    2. TypeError msg ~ Something  (and the other way around)
---    3. C (TypeError msg)          (for any parameter of class constraint)
+--    1. TypeError msg a b c
+--    2. TypeError msg a b c ~ Something (and the other way around)
+--    4. C (TypeError msg a b c)         (for any parameter of class constraint)
 getUserTypeErrorMsg :: Ct -> Maybe Type
 getUserTypeErrorMsg ct
   | Just (_,t1,t2) <- getEqPredTys_maybe ctT    = oneOf [t1,t2]

compiler/types/Type.hs

@@ -707,7 +707,10 @@ isStrLitTy _                    = Nothing
 -- If so, give us the kind and the error message.
 userTypeError_maybe :: Type -> Maybe Type
 userTypeError_maybe t
-  = do { (tc, [_kind, msg]) <- splitTyConApp_maybe t
+  = do { (tc, _kind : msg : _) <- splitTyConApp_maybe t
+          -- There may be more than 2 arguments, if the type error is
+          -- used as a type constructor (e.g. at kind `Type -> Type`).
+
        ; guard (tyConName tc == errorMessageTypeErrorFamName)
        ; return msg }