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>
merge develop -D org -I include -E exclude: PR 528 (Tile performance improvements for VMS and NDPI)
merge develop -D org -I include -E exclude: PR 774 (A reader for the Yokagawa Cellvoyager automated microscope.)
merge develop -D org -I include -E exclude: PR 775 (Add JWlz to Ivy dependencies)
merge develop -D org -I include -E exclude: PR 788 (Do not prompt to update to the latest stable version if Fiji is used)
merge develop -D org -I include -E exclude: PR 790 (Update netCDF to 4.3.19)
merge develop -D org -I include -E exclude: PR 796 (Zip: improve reader and handle performance)
merge develop -D org -I include -E exclude: PR 804 (Leica LIF: 'C' goes before other dimensions if SizeC and SizeT > 1)