[identity] Add HTTP error utilities

Summary:

• Added some handy HTTP error response utils
• Created handle_db_error() counterpart for HTTP

Depends on D12301

Test Plan:

• GET http://localhost:51004/whatever returns 404 as expected
• Tested together in later diffs

Reviewers: will, varun

Reviewed By: will

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12302