StartHeatbeat should send error if resp.Code is not 0

[ranggarmaste]

err = bot.StartHeartbeat() will not return error if StartHeartBeat returns response with non-0 Code, and then it will send TenantAppAccessToken as an empty string

I think we should throw an error instead for the first attempt of GetAccessTokenInternal? I encountered a stupid error of setting up the app_id wrongly and it took me a while to figure out

[crispgm]

This idea looks reasonable. Thanks for the feedback.