[services] [2/n] Replace panic with custom errors on search index lambda

Summary:
This adds custom errors to the lambda function instead of panics.

Previously, we used anyhow for errors and expect when unwrapping result/option types. This has all been replaced by custom error types in a new file called error.rs

Depends on D10788

Test Plan: Tested on staging. Working properly.

Reviewers: varun, bartek

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10736