[identity] Use Debug trait for AWS errors

Summary:
Noticed this while debugging. The Display trait for AWS errors is useless and we should
always use Debug - so do {err:?} instead of {err} in format strings.
Also added messages in two places they were missing.

Test Plan: Got better error log when AWS error occurred.

Reviewers: will, varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12577