rgw-admin: link / unlink should report errors

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: replace logic that compares regions

The logic was a bit broken. Basically, we want to make sure
that region names are the same. However, if region name is not
set then we need to check whether it's the master region. This
can happen in upgrade cases where originally we didn't have
a region name set.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>