NEXUS-5849: Mock was incomplete causing NPE
NEXUS-5849: Test adapted

Essence is same, "connection was refused", but this time it's detected before
nexus transport is consulted (transport error), instead the usual
java ioex message is sent, "connection to XXXX refused".

Assertion adapted accordingly.
NEXUS-5849: improvement for better message handling

The method checking for blacklist is throwing now
exception if needed.