rpc: document returned error fields as optional if applicable

Summary:
Affects the following RPCs:

• analyzepsbt
• signrawtransactionwithkey
• signrawtransactionwithwallet

We do not have the estimate(smart|raw)fee RPCs to update.

This is a backport of core#19644

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10074