Log a fatal exception in debug mode

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Avoid NullPointerException after removing an update site

This prevented the TrackMate-dev update from being populated successfully.
See https://github.com/imagej/imagej-updater/issues/20.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>