ensures the entrypoint exits non-zero when an error happens

[mhrivnak]

fixes #40

output with the change:

[mhrivnak@roadie ]$ podman run --rm -it -v /:/data localhost/gp2g
No database found, intializing gpodder2go ...
2024/12/12 14:51:12 unable to open database file: out of memory (14)
[mhrivnak@roadie ]$ echo $?
1