EZP-22770: Make sure the HTTP response is provided even in case of error

JIRA: https://jira.ez.no/browse/EZP-22770

# Description

This PR makes sure the HTTP response is always provided even in case of error (previously `false` was provided) so that it's possible for custom code to correctly detect what happened.

While fixing this issue, I find out that the Discovery Service was for a half part unused, for the other not really clearly organized nor correctly tested... So this PR also fixes it

# Tests

unit tests + manual tests in the PlatformUI