[web] fix uncaught exception bug

Summary:
Depends on D12547

Looks like this bug was introduced in D11768. We don't need to re-throw here.

Test Plan: no longer getting an uncaught exception error when SIWE fails

Reviewers: inka, ginsu

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12548