NEXUS-5472: NEXUS-5552: Fixing a bug wrt Error sensing

If remote content discoverer detected an error, it was not
signaling it properly (it depended on which strategy reported)
it, causing that NEXUS-5552 gets into wrong state (soft
failure not hard failure), hence retry did not happen either.

This fixes it, by putting the remote discovery into hard failure
state, but also somewhat fixes NEXUS-5552 (at least for me locally)
as the "hourly update job" is started with slight delay from boot,
and successfully updates the WL of Secure Central as async
setup of secure access already happens.
NEXUS-5472: NEXUS-5552: Added UT

The UT asserts that remote content discoverer sets discovery
state to new ERROR state when some error happens during discovery
(transport or unexpected state), that records properly strategy
that failed and gives proper message.