Extend BlockNak

BlockNak can now contain an error message that will be sent as part
of the message.
Fix JITPolicyRule being ignored

JITPolicyRule was called after the StackTraceElementMatcherPolicyRule
which always denied access for cases where JITPolicyRule would kick in
so JITPolicyRule was ignored.
Fix block type handling on module

A block type of null was never accepted. This is now changed so that
null is handled the same as an empty String.
Improve logging in shutdown()

The watcherThread always tried killing the shutdownThread, now it
detects if the thread has already finished and only logs a message
if that's the case.
The shutdownThread will now no longer try to shutdown new blocks if
it was interrupted. This is also logged.