Improve snapshot listing error message

Provide a hint in the error message if listing snapshots for a
single dataset fails.

Using -r is not needed to list all snapshots so requiring it when
listing snapshots for a single dataset makes it confusing. This
change will make the error message more clear.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Md Islam <mdnahian@outlook.com>
Closes #8047