[web] Display an error message when adding new user failed

Summary:
Catch an exception during relationship update and display error message in case of any failure.

Depends on D4315

Test Plan: Modify relationship responders to fail every second time and check if an error is displayed.

Reviewers: def-au1t, atul, yayabosh

Reviewed By: atul, yayabosh

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4364