NEXUS-5472: Improving messaging and selectivity

proxy404 feature supports only maven2 hosted, proxy and
group repositories. Event dispatcher updated to cover
all those cases.

also, messaging is improved, to clearly state why WL is
not published for given repository.
NEXUS-5472: Feature only supports Maven2 hosted, proxy or group reposes

REST resource fixed to throw 400 Bad Request on everything else.

UI fixed to simply not show the WL tab in any other case.