mgr/cephadm: fix host connection exception

mgr/cephadm: fix host connection exception

1- catch exception thrown by _get_connection itself
2- make the error message more informative.  the old one was

Error ENOENT: New host dael (10.3.64.25) failed to connect: `ssh -F /tmp/cephadm-conf-9tvrgrls -i /tmp/cephadm-identity-sj66ado1 root@10.3.64.25`

which was both misleading and had irrelevant information.  Now,

Error ENOENT: Failed to connect to dael (10.3.64.25).  Check that the host is reachable and accepts connections using the cephadm SSH key

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