ERR_HTTP2_MAX_PENDING_SETTINGS_ACK

每当一个 HTTP/2 SETTINGS 帧被发送到连接的对端时，对端需要发送确认其已经收到并应用了新的 SETTINGS。 默认情况下，可以在任何给定时间发送最大数量的未确认 SETTINGS 帧。 当达到该限制时使用此错误码。

Whenever an HTTP/2 SETTINGS frame is sent to a connected peer, the peer is required to send an acknowledgment that it has received and applied the new SETTINGS. By default, a maximum number of unacknowledged SETTINGS frames may be sent at any given time. This error code is used when that limit has been reached.