[keyserver] Log the exception when failing to synchronize links with blobs

Summary: This will help with debugging potential issues.

Test Plan: Kill the keyserver, delete auth metadata, decrement db version, delete the holder and update invite link secret. Then run the keyserver - the migration should succeed by printing an error message and an exception about invalid_credentials.

Reviewers: michal, bartek, inka

Reviewed By: inka

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11088