Tracking Issue for GSP-127: Add ServiceInternel and RequestThrottled Errors

[xxchan]

proposal

• #616 Add the error codes in go-storage.
• beyondstorage/site#138 Add them in the doc.

[Xuanwo]

Thanks to @junaire 's contribution, GSP-127 has been implemented. Our services can use ErrServiceInternel and ErrRequestThrottled now.