patch 8.2.2722: Vim9: crash when using LHS with double index

Problem:    Vim9: crash when using LHS with double index.
Solution:   Handle lhs_dest which is "dest_expr". (closes #8068)
            Fix confusing error message for missing dict item.
diff --git a/src/eval.c b/src/eval.c
index d492649..05b6584 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1474,7 +1474,7 @@
 	{
 	    if (op != NULL && *op != '=')
 	    {
-		semsg(_(e_letwrong), op);
+		semsg(_(e_dictkey), lp->ll_newkey);
 		return;
 	    }