Doesn't check entered recovery key is correct before attempting to decrypt secrets

Eyeballing the code in `MatrixSDK/Crypto/Recovery/MXRecoveryService.m` and `MatrixSDK/Crypto/SecretStorage/MXSecretStorage.m`, and looking at a rageshake, it looks very much as though we don't validate the entered recovery key against the `mac` stored in `m.secret_storage.key.[key ID]` (see https://spec.matrix.org/v1.17/client-server-api/#key-storage).

This means that error reporting to the user is confusing: rather than saying "this is the wrong key", we have much more obscure errors. (Or, in fact, none at all, because it just loops back to "verify your account").