Merge #927: commands: include missing amount in spend response

1339898 commands: include missing amount in response (jp1ac4)

Pull request description:

  This PR follows a discussion around #873 (comment).

  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 #822 (comment)) and instead the spend module's `create_spend` is currently used (see #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

doc/API.md

@@ -180,11 +180,18 @@ This command will refuse to create any output worth less than 5k sats.
 
 #### Response
 
+If the spend is created successfully, the following response will be received:
+
 | Field          | Type              | Description                                          |
 | -------------- | ----------------- | ---------------------------------------------------- |
 | `psbt`         | string            | PSBT of the spending transaction, encoded as base64. |
 | `warnings`     | list of string    | Warnings, if any, generated during spend creation.   |
 
+If there are insufficient funds to create the required spend, then the following response will be received:
+
+| Field          | Type              | Description                                          |
+| -------------- | ----------------- | ---------------------------------------------------- |
+| `missing`      | integer           | Additional sats required to create the spend.        |
 
 ### `updatespend`
 
@@ -295,9 +302,7 @@ allowed in order to replace this transaction using RBF (see https://github.com/b
 
 #### Response
 
-| Field          | Type      | Description                                          |
-| -------------- | --------- | ---------------------------------------------------- |
-| `psbt`         | string    | PSBT of the spending transaction, encoded as base64. |
+The response is the same as for [`createspend`](#createspend).
 
 ### `startrescan`