Improve LOGGER levels in Memoizer

Various conditions in Memoizer were being logged
at DEBUG when they were really error or at least
exceptional states. This corrects most of them.
There is still some question of what should be
at TRACE and what at DEBUG.

Also, one log statement was being reached if the
condition didn't hold. This is now checked by an
if/else.

See:
https://www.openmicroscopy.org/community/viewtopic.php?f=4&t=7548

/cc @melissalinkert -- Happy to review all the log statements if the time is right.