Coin selection: no coins selected if the amount to send over balance amount

[pythcoiner]

but BTC left to select amount looks correct

[jp1ac4]

Currently, if there's an error due to insufficient funds, we don't make any changes to the selected coins.

But I think in such a case where no coins were previously selected, it would make sense to mark all coins as selected. However, there may be some coins shown that were not coin selection candidates (e.g. unconfirmed external deposits). So we would ideally have a way to know which coins had been used by lianad as coin selection candidates without repeating the logic, which could diverge over time.

[pythcoiner]

i thinks it's also fine to not select coins, but in this case we should display a warning "insufficient balance" instead wrong "xxx left to select"

[jp1ac4]

Ah yes. I think that would be better.

There might be a situation where there's insufficient balance using coin selection, but sufficient balance if including some unconfirmed coins manually (if they're external deposits). Maybe we could add something to the message to allow for this possibility, but it would probably be too long and not required most of the time so I think your suggestion makes sense.

[nondiremanuel]

Related to #117