Merge #20403: wallet: upgradewallet fixes, improvements, test coverage

<pre>Merge #20403: wallet: upgradewallet fixes, improvements, test coverage 3eb6f8b2e61c24a22ea9396d86672307845f35eb wallet (not for backport): improve upgradewallet error messages (Jon Atack) ca8cd893bb56bf5d455154b0498b1f58f77d20ed wallet: fix and improve upgradewallet error responses (Jon Atack) 99d56e357159c7154f69f28cb5587c5ca20d6594 wallet: fix and improve upgradewallet result responses (Jon Atack) 2498b04ce88696a3216fc38b7d393906b733e8b1 Don't upgrade to HD split if it is already supported (Andrew Chow) c46c18b788cb0862aafbb116fd37936cbed6a431 wallet: refactor GetClosestWalletFeature() (Jon Atack) Pull request description: This follows up on #18836 and #20282 to fix and improve the as-yet unreleased `upgradewallet` feature and also implement review follow-up in https://github.com/bitcoin/bitcoin/pull/18836#discussion_r519328607. This PR fixes 4 upgradewallet issues: - this bug: https://github.com/bitcoin/bitcoin/pull/20403#discussion_r526063920 - it returns nothing in the absence of an RPC error, which isn't reassuring for users - it returns the same thing both in the case of a successful upgrade and when no upgrade took place - the error message object is currently dead code This PR fixes the above and provides: ...user feedback to not silently return without upgrading ``` { "wallet_name": "disable private keys", "previous_version": 169900, "current_version": 169900, "result": "Already at latest version. Wallet version unchanged." } ``` ...better feedback after successfully upgrading ``` { "wallet_name": "watch-only", "previous_version": 159900, "current_version": 169900, "result": "Wallet upgraded successfully from version 159900 to version 169900." } ``` ...helpful error responses ``` { "wallet_name": "blank", "previous_version": 169900, "current_version": 169900, "error": "Cannot downgrade wallet from version 169900 to version 159900. Wallet version unchanged." } { "wallet_name": "blank", "previous_version": 130000, "current_version": 130000, "error": "Cannot upgrade a non HD split wallet from version 130000 to version 169899 without upgrading to support pre-split keypool. Please use version 169900 or no version specified." } ``` updated help: ``` upgradewallet ( version ) Upgrade the wallet. Upgrades to the latest version if no version number is specified. New keys may be generated and a new wallet backup will need to be made. Arguments: 1. version (numeric, optional, default=169900) The version number to upgrade to. Default is the latest wallet version. Result: { (json object) "wallet_name" : "str", (string) Name of wallet this operation was performed on "previous_version" : n, (numeric) Version of wallet before this operation "current_version" : n, (numeric) Version of wallet after this operation "result" : "str", (string, optional) Description of result, if no error "error" : "str" (string, optional) Error message (if there is one) } ``` ACKs for top commit: achow101: ACK 3eb6f8b MarcoFalke: review ACK 3eb6f8b2e61c24a22ea9396d86672307845f35eb &#128737; Tree-SHA512: b767314069e26b5933b123acfea6aa40708507f504bdb22884da020a4ca1332af38a7072b061e36281533af9f4e236d94d3c129daf6fe5b55241127537038eed</pre>