Define the behavior of VerificationOutcome

[yizha1]

Summary
This is left over from #191 (comment), but focus on the behavior of VerificationOutcome. Currently it contains the only successful signature verification outcome, how about the failure outcome? As an API user, if no failure outcome is returned, how user can troubleshoot the failure of verification. Will this structure be connected to debug option as well?

[yizha1]

As discussed in the community meeting on Nov 4, 2024, move this issue to the future milestone.