[keyserver] Validate SIWEMessage signature in siweAuthResponder

Summary:
Attempt to validate the signature for the SIWEMessage and handle potential errors (unrelated but noticed` siwe` 2.x has much more descriptive error types: https://blob.sh/00a56f.png).

Depends on D6032

Test Plan: Haven't tested just yet but will hardcode some values to try to hit the error cases and make sure things are handled properly.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6033