[herald] Always print the staking reward winner for invalidated blocks

Summary: Even if the guessed reject reason has nothing to do with the staking rewards. In the event it cannot be determined (e.g. there is no such output) it simply prints 'unknown'.

Test Plan:

npm run test

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17040