dm thin: fail messages with EOPNOTSUPP when pool cannot handle messages

Use EOPNOTSUPP, rather than EINVAL, error code when user attempts to
send the pool a message.  Otherwise usespace is led to believe the
message failed due to invalid argument.

Reported-by: Zdenek Kabelac <zkabelac@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>