better error message for the ai command

src/GF/Command/Commands.hs

@@ -644,7 +644,7 @@ allCommands cod env@(pgf, mos) = Map.fromList [
                                                                           else space $$
                                                                                text "fun" <+> vcat [ppCId fid <+> colon <+> ppType 0 [] ty 
                                                                                                        | (fid,ty) <- functionsToCat pgf id])
-                                             Nothing   -> do putStrLn "unknown identifier"
+                                             Nothing   -> do putStrLn ("unknown category of function identifier "++show id)
                                                              return void
          [e]         -> case inferExpr pgf e of
                           Left tcErr   -> error $ render (ppTcError tcErr)

testsuite/runtime/typecheck/typecheck.gfs.gold

@@ -7,10 +7,7 @@ A function type is expected for the expression 1 instead of type Int
 Couldn't match expected type Int -> Int␍
        against inferred type Int␍
 In the expression: 1␍
-␍
-␍
-Function unknown_fun is not in scope␍
-␍
+unknown category of function identifier unknown_fun␍
 ␍
 Category unknown_cat is not in scope␍
 Cannot infer the type of expression \x -> x␍