Display relevant error if SYNC_REPO not set properly
If ones does not read the doc, and SYNC_REPO is not set, then the container will stop with the following error:
koha_1 | Cannnot open file No such file or directory at /kohadevbox/misc4dev/cp_debian_files.pl line 35.
koha_koha_1 exited with code 2
We could do better.
This has been reported at least twice, on the list and IRC.