ceph: messenger: check prepare_write_connect() result

prepare_write_connect() can return an error, but only one of its
callers checks for it.  All the rest are in functions that already
return errors, so it should be fine to return the error if one
gets returned.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>