PSBT Dialogue should check testmempoolaccept

[fanquake]

Right now the user will be presented with a "submit transaction" dialogue even if the utxos don't exist, provided the PSBT checks out. The submit button should probably be disabled with error message shown if it cannot be put into the mempool at the time of loading.

Moved from: https://github.com/bitcoin/bitcoin/issues/19433

[Sjors]

That sounds good, as long as it's still possible to copy the result. An offline signer might not be fully synced.