[web] Avoid changing height when error occurs in log in screen

Summary:
This diff hardcodes the error to have height: 0, and increases the margin around OrBreak to make up for that. I don't love that asymmetry of this solution when displaying the error, but the extra spacing doesn't look too bad.

OrBreak is used in one other place (SIWELogInForm), so I included a screenshot of that screen in the Test Plan.

Depends on D12086

Test Plan:

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12087