[terraform] Add cloudwatch alert for search index lambda failures

Summary: This adds a cloudwatch alert for when there are search index lambda errors. An email notification is sent to error-report@comm.app

Test Plan: Tested on staging with my own email will@comm.app and received alarm email

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11139