Give a better message from 'zpool get' when an invalid pool name is provided

[pcd1193182]

Motivation and Context

If you do zpool get name foo, but you have no pool named "foo", the error message you get is very confusing:

$ zpool get name foo
missing pool name.
...

This error message is pretty confusing and misleading, given that a pool name was provided. It just happens to not be the name of an existing pool. We should give a clearer error message.

Description

We give a clearer error message:

$ zpool get name foo
Cannot get properties of foo: no such pool available."

We also no longer print the usage in that case, since it takes up a lot of space and makes it hard to see the error message. The error message itself makes it clear what is expected in that spot.

How Has This Been Tested?

Manual testing.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[pcd1193182]

The failing automated tests seem unrelated to me.