Raise EOS unsuccessful request reporting to error

When we fail to send a message to the backend we are entering an
inconsistent state, where our users think the candidate is registered,
but the backend knows nothing about it -- which warrants an ERROR
level output.

Also place a FIXME on operations which should be retried.

JIRA: CONTROLLER-1904
Change-Id: I0799afe2e0786468cdef5f80c1064cb08d3c1af0
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>