[lib] fix callServerEndpoint returning undefined

Summary:
This is fix for some serious issue - context in ENG-3806.
When policies were not accepted this function was returning undefined instead of throwing an error to inform the caller that the request did not succeed.

Test Plan: Test policy acknowledgment flow on web, native, and desktop in multiple scenarios.

Reviewers: tomek, michal, ashoat

Reviewed By: ashoat

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7741