rgw.py: add a better error message
rgw.py: add a better error message

When pulling region info from the config
structure, if the region info isn't there,
log a more helpful message.

Signed-off-by: Joe Buck <a95e51950b61b017a762bcc8a962bac0b1a073a0@gmail.com>

SUSE/teuthology,yghannam/teuthology,zhouyuan/teuthology,SUSE/teuthology,t-miyamae/teuthology,tchaikov/teuthology,dreamhost/teuthology,robbat2/teuthology,t-miyamae/teuthology,zhouyuan/teuthology,dmick/teuthology,yghannam/teuthology,tchaikov/teuthology,dmick/teuthology,michaelsevilla/teuthology,ktdreyer/teuthology,ivotron/teuthology,robbat2/teuthology,ivotron/teuthology,SUSE/teuthology,ceph/teuthology,dmick/teuthology,caibo2014/teuthology,michaelsevilla/teuthology,ktdreyer/teuthology,ceph/teuthology,dreamhost/teuthology,caibo2014/teuthology