[native] Handle errors with SIWE WebView
Summary:
More context in this comment: ENG-2544.
The current error message isn't great, so this diff replaces it with an alert.
Test Plan: For testing I forced getSIWENonceCallFailed to be always true and checked if the alert is displayed and if it returns to the login screen when you press "OK".
Reviewers: atul, tomek, kamil
Reviewed By: tomek
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D6097