13398982534d56a5723dfa86723c5917483c8653 commands: include missing amount in response (jp1ac4) Pull request description: This PR follows a discussion around https://github.com/wizardsardine/liana/pull/873#issuecomment-1886715468. The GUI uses the `InsufficientFunds` error to get the missing amount when the user is creating a new spend, but it is not straightforward to extract this information in a general way from the RPC error (see https://github.com/wizardsardine/liana/issues/822#issuecomment-1836482355) and instead the spend module's `create_spend` is currently used (see https://github.com/wizardsardine/liana/pull/863). With this PR, the missing amount will be included in the `createspend` response rather than as an error. These changes are based on suggestions from @darosior and @edouardparis. In a follow-up PR, the GUI should revert to using the `createspend` command to calculate the amount left to select. ACKs for top commit: darosior: re-ACK 1339898 Tree-SHA512: bf702d6b355339e96e719c1d95824e7941ac4fbaece4ec4cccd00b56ea4683ce7fb0cefc43faa5731b57e7935ef99da3a2c73b84aaeb9fa5f67703c799be2196