Exit out if 'exec' fails

[matyasselmeci]

I ran into two issues while trying to use this image with apptainer run:

• apptainer run put -- into the first argument, causing exec "$@" to fail because it wasn't a real option
• the exec command failing did not cause the script to stop

[matyasselmeci]

OK, ready for review. Turns out apptainer wasn't doing anything weird, it's just I was doing apptainer run image.sif --contain ... instead of apptainer run --contain image.sif ... and I was getting what I asked for. We should still handle exec failing.

[brianhlin]

LGTM