rpc: loadtxoutset should return errors that currently go to logs

[Sjors]

Please describe the feature you'd like to see added.

loadtxoutset should return (more of) the errors that currently only appear in the log.

Is your feature related to a problem, if so please describe it.

Currently most errors encountered during loadtxoutset can only be found via the log.

Describe the solution you'd like

No response

Describe any alternatives you've considered

No response

Please leave any additional context

Errors could be shown by loadtxoutset status if the method is made async (see #28620), otherwise just returned.

[pablomartin4btc]

Concept ACK.

eg. currently when re-try already processed and completed loadtxoutset on mainnet the returned error message is:

./src/bitcoin-cli -datadir=${AU_DATADIR} loadtxoutset ${AU_DATADIR}/utxo-800000.dat
error code: -32603
error message:
Unable to load UTXO snapshot .../.test_utxo_2/utxo-800000.dat

While in the logs a user would see:

2023-10-09T20:26:13Z [httpworker.2] [snapshot] waiting to see blockheader 00000000000000000002a7c4c1e48d76c5a37902165a270156b7a8d72728a054 in headers chain before snapshot activation
2023-10-09T20:26:13Z [httpworker.2] [snapshot] can't activate a snapshot-based chainstate more than once

I like also the idea to extend the rpc command with the status param: loadtxoutset status, in order to see percentage of processing, errors or warnings, especially if we manage to make it async.

[brunoerg]

Perhaps a good first issue?

[Sjors]

This shouldn't be closed yet. #28652 only fixes one failure mode.