nfs/blocklayout: Report only when /no/ device is found

Since commit f931d837 ("nfs/blocklayout: refactor block device
opening"), an error is reported when no multi-path device is found.
But this isn't a fatal error if the subsequent device open is
successful. On systems without multi-path devices, this message
always appears whether there is a problem or not.

Instead, generate less system journal noise by reporting an error
only when both open attempts fail. The new error message is more
actionable since it indicates that there is a real configuration
issue to be addressed.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>