patch 9.0.1576: users may not know what to do with an internal error

Problem:    Users may not know what to do with an internal error.
Solution:   Add a translated message with instructions.
diff --git a/src/testdir/test_listdict.vim b/src/testdir/test_listdict.vim
index 877bb80..ac1f29b 100644
--- a/src/testdir/test_listdict.vim
+++ b/src/testdir/test_listdict.vim
@@ -1384,7 +1384,7 @@
   call v9.CheckLegacyAndVim9Failure(['VAR l = [1, 2, 3]', 'LET l[1.1] = 4'], ['E805:', 'E1012:', 'E805:'])
   call v9.CheckLegacyAndVim9Failure(['VAR l = [1, 2, 3]', 'LET l[: i] = [4, 5]'], ['E121:', 'E1001:', 'E121:'])
   call v9.CheckLegacyAndVim9Failure(['VAR l = [1, 2, 3]', 'LET l[: 3.2] = [4, 5]'], ['E805:', 'E1012:', 'E805:'])
-  call v9.CheckLegacyAndVim9Failure(['VAR t = test_unknown()', 'echo t[0]'], ['E685:', 'E909:', 'E685:'])
+  call v9.CheckLegacyAndVim9Failure(['VAR t = test_unknown()', 'echo t[0]'], ['E340:', 'E909:', 'E340:', 'E685:'])
 endfunc
 
 " Test for a null list