better error message if pvfs2-client isn't running

[carns]

If someone tries to mount pvfs without first starting pvfs2-client, this is the error message that shows up in dmesg:

[333190.980150] pvfs2: pvfs2_get_sb -- wait timed out; aborting attempt.
[333190.980161] pvfs2_get_sb: mount request failed with -110

It would be nice if the message were more helpful. Maybe we can check to see if the /dev/ file is open. If not we can add an extra message in this case suggesting the user make sure it is started.