Use BCLog::ANCHORING log category for ContextualValidateAnchor errors

[Jouzo]

What kind of PR is this?:

/kind chore

What this PR does / why we need it:

Use BCLog::ANCHORING log category for ContextualValidateAnchor errors.

[prasannavl]

Would prefer to not eliminate the logs entirely. Better outcome would be if we can keep the errors, when there's a definite mismatch, and then for blocks that aren't in yet, turning them into warnings, that are output once (or if we can do the former cleanly, then possibly don't need to display warnings at all, since these will become errors when the future block is validated)