Compact and renumber `ReportError`

[jcjones]

github.com/ietf-wg-ppm/draft-ietf-ppm-dap/pull/786

Janus (~messages/src/lib.rs:2465) has task_expired(7), invalid_message(8), report_too_early(9), task_not_started(10);

draft-19 is invalid_message(7), report_too_early(8), unknown_verification_key_id(9), unsupported_extension(10).

So -- task_expired and task_not_started are gone, folded into report_dropped(3).

Audit these, fix it up.