refs/reftable: print errors on compaction failure

When git-pack-refs(1) fails in the reftable backend we end up printing
no error message at all, leaving the caller puzzled as to why compaction
has failed. Fix this.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/reftable-backend.c

@@ -1221,8 +1221,12 @@ static int reftable_be_pack_refs(struct ref_store *ref_store,
 		stack = refs->main_stack;
 
 	ret = reftable_stack_compact_all(stack, NULL);
-	if (ret)
+	if (ret < 0) {
+		ret = error(_("unable to compact stack: %s"),
+			    reftable_error_str(ret));
 		goto out;
+	}
+
 	ret = reftable_stack_clean(stack);
 	if (ret)
 		goto out;