Provide useful debug information when an invalid soroban settings upgrade is used

[sisuresh]

At the moment, dumpProposedSettings returns the message configUpgradeSet is missing or invalid if there's an issue with the proposed settings. The upgrade command also just returns Error setting configUpgradeSet on error. We should instead mention which setting is invalid.

[sisuresh]

The restoreTx is missing here, which is only hit if you don't use --signtxs with get-settings-upgrade-txs. This should be fixed as well.