Document common error codes

[wallyqs]

Some of the most common ones

• 503 no responders (like when JS means: quorum temporarily unavailable, JS warming up during boot, otherwise means no subscriber // service available to respond)
• 408 request timeout (too many // 512 fetch requests have timed out on a pull based consumer without receiving messages)
• BadRequestError: code=400 err_code=10099 description='multiple non-filtered consumers not allowed on workqueue stream