internal/postgres: return error from Transact
internal/postgres: return error from Transact

Somehow we lost the error return from the Transact call in saveModule.
This restores it.

Change-Id: I5d0f2d86ccc78a7506307e1d13a475930e36afa5
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/750954
CI-Result: Cloud Build <ea5729d4cf3392b9d382b6ca272c65aa27554f44@system.gserviceaccount.com>
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite