The INSUFFICIENT_UTXO error is not clear enough

[ShookLyngs]

Issue

The reasons triggering the INSUFFICIENT_UTXO error are clear:

1. Not enough satoshi collected to cover the needs of the outputs and fee
2. Not enough satoshi collected to generate a change output (change output value < minUtxoSatoshi)

However, it's hard to prompt a specific reason that causes the error because there could be indirect causes:

1. Small value UTXOs (value < minUtxoSatoshi) of an address will be excluded as they're considered dust
2. If ckbVirtualResult.needPaymasterCell == true, there's an extra payment to the paymaster
3. The change output's defined value must be >= minUtxoSatoshi

Possible Resolutions

• I think this is somewhat a lack-of-documentation issue, and if we can provide a better version of the documentation, things could be easier. For starters, we could write a "Common Errors" section in the documentation, explaining what the errors actually mean in reality and guiding users to find the actual cause or reason for the error.
• For the indirect cause 3 mentioned above, we could add an option to allow treating the change value as fee if the value is smaller than the minUtxoSatoshi, instead of always generating a change output for that. Related issue: Don't create change output (uneconomical output) if not worthy #183.

[Flouse]

• For the indirect cause 3 mentioned above, we could add an option to allow treating the change value as fee if the value is smaller than the minUtxoSatoshi, instead of always generating a change output for that. Related issue: Don't create change output if not worthy #183.

Ideas about making the BTC tx cost as low as possible

1. 546 <= changeValue < minUtxoSatoshi
   create the change output directly with changeValue.

2. changeValue < 546
   The change could be treated as a fee, as it may not be economically viable to create a change output for such a small amount.
   => Don't create change output (uneconomical output) if not worthy #183