[identity] improve error log for failed backup deletion

Summary:
We've noticed that sometimes the response body is empty. Improving this error log to hopefully uncover some more information if we see another backup deletion call fail with no response body

Depends on D13498

Test Plan: disabled local backup service and confirmed that the HTTP response code appears in log

Reviewers: bartek, will

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13499