Voucher subscriber observability

When running Voucher/v2 in subscriber mode there is minimal logging as to what went wrong when a vouch fails. More logging needs to be added to [check.go](https://github.com/grafeas/voucher/blob/25c71871a2ac9abc5f7c7dcd00d4fc39a8f71ef1/v2/subscriber/check.go#L50-L54) and subsequent functions called.

Start in v2/subscriber/check.go at lines 50–54 and trace the subsequent functions called when a vouch fails. Identify where subscriber-mode failures lose diagnostic context, then add logging at those points so failures provide useful details. Done means failed vouches in Voucher/v2 subscriber mode have more actionable logs.

Written by the indexing model from the issue text.