gateway_balances
* When `account` does not exist in the ledger, return `actNotFound`
* (Previously, a normal response was returned)
* Fix#4290
* When required field(s) are missing, return `invalidParams`
* (Previously, `invalidHotWallet` was incorrectly returned)
* Fix#4548
channel_authorize
* When the specified `key_type` is invalid, return `badKeyType`
* (Previously, `invalidParams` was returned)
* Fix#4289
Since these are breaking changes, they apply only to API version 2.
Supersedes #4577