mount.nfs: make nfs_{nfs,mount}_proto set errno on error

Have nfs_nfs_proto and nfs_mount_proto set errno to EPROTONOSUPPORT on
error. This helps default_value to display sane warning messages.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>