course_plot_processing.php: simplify existence check
course_plot_processing.php: simplify existence check

Since we need to construct the SmrSector objects anyway, perform the
existence check by catching a `SectorNotFoundException` instead of
finding the galaxy that contains the sector.

smrealms/smr,smrealms/smr,hemberger/smr,smrealms/smr,hemberger/smr,hemberger/smr,hemberger/smr