LUD-03: Provide recommendation for zero balance withdrawal response

[wbobeirne]

LUD-03 should provide an explicit recommendation for what to do if a requester has no amount they could withdraw. Right now someone could choose to implement this as:

1. minWithdrawable and maxWithdrawable as 0
2. Return an {"status": "ERROR"} error

While either approach should probably be handled from clients, it would be nice to have an expectation around this and reduce ambiguity.

As an aside, it might also be wise to specify that the *Withdrawable amounts should never be negative, unless there's a case for that that I'm not thinking of.

[hsjoberg]

I think it makes sense to set min/maxWithdrawable to 0, instead of returning an error.

This would mean LUD-14 and LUD-19 are still discoverable.