monc: only warn about missing keyring if we fail to authenticate

monc: only warn about missing keyring if we fail to authenticate

This avoids the situation where a librados or other user with the default
of 'cephx,none' and no keyring is authenticating against a cluster with
required of 'none' and an annoying warning is generated every time.  Now
we only print a helpful message if we actually failed.

Signed-off-by: Sage Weil <sage@inktank.com>