[web] Clear error message before sending another request on settings page

Summary: When a request is pending we would like to clean the error message so that when a new request fails a user is able to see that this is another request.

Test Plan: Modify update_thread endpoint to fail every second time and check if the UI behaves correctly

Reviewers: def-au1t, atul, yayabosh

Reviewed By: atul, yayabosh

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4400