add xhub verification middleware and error handling
add xhub verification middleware and error handling

DJ1K1/botkit,howdyai/botkit,DJ1K1/botkit,howdyai/botkit,samuelsan/YelpBot,ouadie-lahdioui/botkit,healthchatnow/hcn,howdyai/botkit,EliKrumholz/botkit,howdyai/botkit,acrollet/botkit,dhulser/slack-reporting-bot