Be more verbose in radosgw init script issues, return the right exit codes

Ref: http://tracker.ceph.com/issues/6710
Before, we would exit 0 where we should exit 1 or use "continue".
This means we were not returning exit 1 if there was an error and
radosgw did not start.
This commit aims to handle theses cases better.
We also now verify that radosgw started successfully.