Print error and exit if we can't get commit id

mirage/unikernel.ml

@@ -715,7 +715,10 @@ module Make
 
     let commit_id git_kv =
       Store.digest git_kv Mirage_kv.Key.empty >|= fun r ->
-      Result.get_ok r
+      Result.fold r ~ok:Fun.id
+        ~error:(fun e ->
+            Logs.err (fun m -> m "%a" Store.pp_error e);
+            exit 2)
 
     let repo commit =
       let upstream = List.hd (String.split_on_char '#' (Key_gen.remote ())) in