[Improvement] Prevent burning 5 XRP Fee for an invalid AccountDelete

[WietseWind]

When sending an AccountDelete, 5 XRP (fee) will be burned even if the action is invalid. The most common reason for the AccountDelete to be invalid is tecTOO_SOON:

"Occurs if the sender's Sequence number is too high. The transaction's Sequence number plus 256 must be less than the current Ledger Index."

A common reason for this to happen is when one deletes an account, funds it again and tries to delete it again relatively shortly after deleting it the first time around.

We can easily detect that upfront, before sending (and thus allowing the XRPL to burn) the 5 XRP fee.

Pseudo code (sample: Testnet: rHB9yje7AJ7VrYxwmMXQQJNYWjXUduuv57)

• The account (to be deleted) sequence is 15597090
• The Ledger Index is 15597418
• The account (to be deleted) sequence + 256 = 15597090 + 256 = 15597346
• 15597346 (account sequence) > 15597418 (ledger index) so in this example: 15597346-15597418 = -72, which is negative. It takes Abs(72) = 72 more ledgers to close before this account can be deleted again.

Proposed change

When XUMM opens a Sign Request for an AccountDelete and this ^^ is the case, we should hide the existing red warning message and hide the Slide to Sign button and instead show a message:

This account cannot be deleted (yet). It takes {Abs(...)} more ledgers to close before this account can be deleted.