[Identity] Add new Error variant

Summary:
The Rusoto Error variant is too specific to handle both get and put errors. I'm therefore breaking it out into two variants, RusotoGet and RusotoPut. This will help us handle errors related to the put_item Rusoto method.

Depends on D3960

Test Plan: cargo build

Reviewers: jimpo, karol-bisztyga, palys-swm

Reviewed By: jimpo, karol-bisztyga

Subscribers: ashoat, Adrian, yayabosh, atul

Differential Revision: https://phabricator.ashoat.com/D3961