If one output has amount below dust, throw exception instead of removing it

[NicolasDorier]

While it shouldn't really matter economically speaking to remove output with a value that is too small, it is surprising behavior for users, and give rise to several edge case.

Assuming 540 of dust, here is a summary of the changes of behavior:

Case	Before	Now
you send bob 600 satoshi, but substract 400 sats, the output left is 200 sats.	Silently remove output	Throw exception
you send bob 200 satoshi (without substract fee)	Silently remove output	Throw exception
you sent bob 600 satoshi, substracted 700 sats of fee	Throw exception	Throw exception

Before, we would silently remove such outputs in the built transaction. Now the builder will always throw an exception.

[farukterzioglu]

this error is out of if (SubstractFee block. But it says Can't substract fee

This error will be thrown even user tries to send w/o SubstractFee, if the amount is less than dust. Kinda mis-leading.

[farukterzioglu]

How can we differentiate the cases;

• User tries to send lower than dust (w/ or w/o fee subtract)
• User tries to send bigger than dust, but the amount after fee subtraction is less than dust

If we can differentiate the second case, user can try to send it again after fees dropped.
When the fees are lower, subtracted amount will be less and remaining amount could be bigger than dust.

But the first case will be always fail.

Right now both of them throws OutputTooSmallException exception, so no way to handle them differently