replicate_osm_file.sh: Don't exit with negative status.

Exit statuses fall between 0 and 255.

Exit using the osmosis status in case it fails instead of -1.

This issue was reported in [Debian Bug#772355](https://bugs.debian.org/772355)
Merge pull request #15 from sebastic/debian-bug772355

replicate_osm_file.sh: Don't exit with negative status.