[web] Show generic Alert if an error occurs during QR code auth

Summary:
This matches behavior on native. It's better to show something than to silently fail.

Depends on D12125

Test Plan: I hardcoded a failure in the web implementation of IdentityServiceClient and saw this error:

Reviewers: bartek

Reviewed By: bartek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12126