ensure InterruptedException handling

Actually this breaks and thus the method continues (only logging, no throwing here) so that nulling out does still occur.

I'm actually concerned a bit about the opposite, which is the fact that this may prematurely return before the server is really shutdown and then log the potentially incorrect statement:

````
logger.info("HSQL Server Shutdown completed");
````

Perhaps that log statement needs to be conditional upon the value of server.getState()