NEXUS-5472: Lowering log level

To not spam about task started/stopped at INFO level.
Cancellation and interruption is still left at INFO.
NEXUS-5472: Added new discovery state

Do differentiate between "failed" (cannot get WL from remote) and
"error" (some error occurred during getting WL from remote). This
is important distinction as the updates for ERROR state should
be commenced on next invocation of mayUpdateProxyWhitelist()
method.

Also, mayUpdateProxyWhitelist() method was fixed, as it was wrongly
always trying remote discovery, for FAILED proxies. Again, FAILED
ones should be retried by configured interval.

WLManager "boot" sequence unchanged by is prepared for
eventual delay of proxy initialization on boot (as we saw
Secure Central and this are fighting a bit).

In general, now exceptions are more properly handled, and will
be recorded using new state.