[web] fix uncaught exception bug

Summary: we don't need to re-throw here. i think this was introduced in D11765, when we introduced new login hooks.

Test Plan: no longer getting an uncaught exception error when i enter an incorrect password

Reviewers: ashoat, inka

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12471