[keyserver] Check if a blob exists

Summary:
After fetching a blob, check if it exists, and return different result depending on the test.

Depends on D9942

Test Plan: Create a script that tries to download two blobs - one with existing hash and one without. Check if the result is correct.

Reviewers: kamil, marcin, bartek

Reviewed By: marcin

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9943