[keyserver] Change blob upload functions to return more usable results

Summary:
We would like to be able to differentiate between hash in use and a plain failure and to achieve that we need a different API

Depends on D10099

Test Plan: Run a script and check if both SUCCESS and HASH_IN_USE results are returned

Reviewers: michal, bartek, rohan, marcin

Reviewed By: bartek, marcin

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10131