Provide a better message to the user if they need to enable additional bot intents

[Cldfire]

In #5, it was noted that forgetting to enable the SERVER MEMBERS INTENT results in seemingly unrelated error messages:

Failed to update bot's status: sending the message over the websocket failed

It would be great if we could identify that the user hasn't enabled a needed intent and warn them about it on startup, along with steps to fix the situation.