Return an exit code for configuration errors

Red Hat's QA noticed that in case of the infamous "xattr>=0.4"
error, swift-init exits with a zero error code, which confuses
startup scripts (not Systemd though -- that one knows exactly
if processes run or not).

The easiest fix is to return the error code like Guido's blog
post suggested.

Change-Id: I7badd8742375a7cb2aa5606277316477b8083f8d
Fixes: rhbz#1020480
Merge "Return an exit code for configuration errors"