[keyserver] edit warning to notify user of optional login

Summary: Edit console warning message for a failed identity service login to notify user that it is optional as the staging environment for devs is not yet setup.

Test Plan: - confirm message when starting keyserver

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek, wyilio, varun

Differential Revision: https://phab.comm.dev/D9347