internal/v4: fix search error handling

We use the new HandleJSON API which doesn't give a ResponseWriter
to use.
It's explicitly documented that we omit metadata when it's not relevant to the entity (see https://docs.google.com/a/canonical.com/document/d/1TgRA7jW_mmXoKH3JiwBbtPvQu7WiM6XMrz1wSrhTMXw/edit#bookmark=id.uu3ojqeyq0b8). That is already catered for by GetMetadata. This case we're considering is different, I feel, because our database is internally inconsistent - this particular charm or bundle is actually unusable because the archive doesn't exist. If we were to include the entity, it would have no metadata at all, which wouldn't really be very useful.