patch 9.0.0335: checks for Dictionary argument often give a vague error

Problem:    Checks for Dictionary argument often give a vague error message.
Solution:   Give a useful error message. (Yegappan Lakshmanan, closes #11009)
diff --git a/src/list.c b/src/list.c
index a6805ed..7bc34db 100644
--- a/src/list.c
+++ b/src/list.c
@@ -2236,11 +2236,8 @@
     if (argvars[2].v_type != VAR_UNKNOWN)
     {
 	// optional third argument: {dict}
-	if (argvars[2].v_type != VAR_DICT)
-	{
-	    emsg(_(e_dictionary_required));
+	if (check_for_dict_arg(argvars, 2) == FAIL)
 	    return FAIL;
-	}
 	info->item_compare_selfdict = argvars[2].vval.v_dict;
     }