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/userfunc.c b/src/userfunc.c
index 5b2c876..c30c352 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -3247,7 +3247,7 @@
 restore_funccal(void)
 {
     if (funccal_stack == NULL)
-	iemsg("INTERNAL: restore_funccal()");
+	internal_error("restore_funccal()");
     else
     {
 	current_funccal = funccal_stack->top_funccal;