patch 8.2.1430: Vim9: error for missing comma instead of extra white space

Problem:    Vim9: error for missing comma instead of extra white space.
Solution:   Check if comma can be found after white space. (closes #6668)
            Also check for extra white space in literal dict. (closes #6670)
diff --git a/src/list.c b/src/list.c
index 14a31bc..212d827 100644
--- a/src/list.c
+++ b/src/list.c
@@ -1219,7 +1219,12 @@
 	if (!had_comma)
 	{
 	    if (do_error)
-		semsg(_("E696: Missing comma in List: %s"), *arg);
+	    {
+		if (**arg == ',')
+		    semsg(_(e_no_white_before), ",");
+		else
+		    semsg(_("E696: Missing comma in List: %s"), *arg);
+	    }
 	    goto failret;
 	}
     }