[keyserver] return status code from blob download function

Summary:
In order to check if the blob service has a farcaster channel tag blob or not, we need the status code to determine if it was a 404.

Depends on D13762

Test Plan: flow check. Tested this later in the stack by checking the status code

Reviewers: varun, bartek, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13773