patch 9.0.1723: Fix regression in {func} argument of reduce()

Problem: Fix regression in {func} argument of reduce()
Solution: pass function name as string again

Before patch 9.0.0548, passing a string as {func} argument of reduce()
is treated as a function name, but after patch 9.0.0548 it is treated as
an expression instead, which is useless as reduce() doesn't set any v:
variables. This PR restores the behavior of {func} before that patch.

Also correct an emsg() call, as e_string_list_or_blob_required doesn't
contain format specifiers.

closes: #12824

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

src/strings.c

@@ -1032,7 +1032,7 @@ string_reduce(
 	    break;
 	len = (int)STRLEN(argv[1].vval.v_string);
 
-	r = eval_expr_typval(expr, argv, 2, fc, rettv);
+	r = eval_expr_typval(expr, TRUE, argv, 2, fc, rettv);
 
 	clear_tv(&argv[0]);
 	clear_tv(&argv[1]);