Withdraw MAX fails for SIA while competing unconfirmed transaction

[smk762]

To Reproduce

1. Login, go to SCZEN page
2. Withdraw some SCZEN
3. Soon after, withdraw MAX the remaining SCZEN
4. See error

Screenshots

Expected behavior
An error which makes sense and is actionable, or a successful withdraw op.

Found in
0.9.3.zzz RC

Additional context

• The error also reveals the rpc password. This should be redacted. Ideally, a human error message would be preferable.
• The root cause of this error is a limitation on https://siascan.com/ which effectively means that KDF has no awareness of confirmed/unconfirmed balance. As appears in the coin page, until a transaction is confirmed, the balance will not change. This results in an incorrect assumption of the value of "max"