don't crash nouveau_index_updater for any DOWN Reason

Hard to induce in a unit test but under stress it is possible to get back
Reason's like `{timeout, {gen_server, call, ...` among others. Pass these
along to the client processes and do not crash the manager.

src/nouveau/src/nouveau_index_manager.erl

@@ -91,22 +91,22 @@ handle_info({'DOWN', IndexerRef, process, _Pid, Reason}, State) ->
  true = ets:delete(?BY_REF, IndexerRef),
  [{_, Index, Queue0}] = ets:lookup(?BY_DBSIG, DbSig),
  {{value, From}, Queue1} = queue:out(Queue0),
- case Reason of
- ok ->
- gen_server:reply(From, ok);
- {error, Msg} ->
+ if
+ Reason /= ok ->
  couch_log:error(
  "~p: db:~s ddoc:~s index:~s failed with: ~p",
  [
  ?MODULE,
  mem3:dbname(Index#index.dbname),
  Index#index.ddoc_id,
  Index#index.name,
- Msg
+ Reason
  ]
- ),
- gen_server:reply(From, {error, Msg})
+ );
+ true ->
+ ok
  end,
+ gen_server:reply(From, Reason),
  case queue:is_empty(Queue1) of
  true ->
  true = ets:delete(?BY_DBSIG, DbSig);