NOJIRA:  Enhancements to logging & error handling related to how DLM dec...

...ides whether a user receives a fragment;  should make troubleshooting much clearer

When issues occur, the current code produces stack traces in the logs that don't name fragments, groups, or user accounts.  These omissions make it hard to narrow your search for the cause of an issue, such as a missing group.
This gating `LOG.isDebugEnabled()` is unnecessary since the gated debug message is not dynamic and so is not expensive to generate.  Preferable to go ahead and just call

    LOG.debug("isApplicable()=false due to evaluators collection being null");

forthwith for code readability. `LOG.debug()` can no-op if it's below the logging threshold.