send: refactor CreateTransaction flow to return a BResult<CTransactionRef>

Summary:
Partial backport of core#25218 (https://github.com/bitcoin/bitcoin/pull/25218/commits/22351725bc4c5eb63ee45f607374bbf2d76e2b8c) and core#25721.

We jump to use util::Result directly as it's already in the repo and is cleaner.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17956