Poorly worded error message when koha directory is not found / undefined.

If SYNC_REPO is incorrectly defined or not defined, fails with a confusing error message:

 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

Should instead fail with an error making it clearer that SYNC_REPO is not correctly defined or undefined.

Thanks for Joubu for helping me identify this issue!