CellVoyagerReader: better handling of possible null service

Complaint from findbugs, reported by Josh Moore:
"findbugs is complaining (correctly) that omeMD could be null here if
service is null:
http://hudson.openmicroscopy.org.uk/view/Failing/job/BIOFORMATS-merge-develop/616/findbugsResult/new/HIGH/
Likely this whole block (481-514) could be simplified down to a single
try block which throws if omeMD is left null."

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>