Use consistent error message in zpool sub-command

The zpool sub-commands like iostat, list, and status should
display consistent message when a given pool is unavailable or
no pool is present.  This change unifies the default behavior
as follows:

  root@prasad:~# ./zpool list 1 2
  no pools available
  no pools available

  root@prasad:~# ./zpool iostat  1 2
  no pools available
  no pools available

  root@prasad:~# ./zpool status 1 2
  no pools available
  no pools available

  root@prasad:~# ./zpool list tan 1 2
  cannot open 'tan': no such pool

  root@prasad:~# ./zpool iostat tan 1 2
  cannot open 'tan': no such pool

  root@prasad:~# ./zpool status tan 1 2
  cannot open 'tan': no such pool

Reported-by: Rajshree Thorat <rthorat@stec-inc.com>
Signed-off-by: Prasad Joshi <pjoshi@stec-inc.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #306