[services][blob] Add custom errors to db methods

Summary:
Continuation of adopting custom errors. This diff replaces most of anyhow errors with dedicated structures.

Depends on D5857

Test Plan: Artifically caused a few errors on purpose to see how they are handled.

Reviewers: varun, tomek, jon, michal

Reviewed By: varun, jon

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D5858