Fix incorrect error message for arguments with choices

[fralik]

• If user provided an invalid value for an argument with a list of choices, then the error message will not indicate this. Rather it will indicate that cli command group is invalid. This is a quick fix and I am not sure that it doesn't break Knack. From my tests, this fix is better than removing overridden _check_value method.

Fixes #236

[ghost]

All CLA requirements met.

[jiasli]

@fralik, Thanks a lot for the contribution and this is a nice finding!

The corresponding fix in Azure CLI is done by Azure/azure-cli#6636. The change is later backported to knack by #146, but the logic for invalid argument value handling got lost during the backporting.

As the logic of _check_value in Azur CLI has become rather complex due to addition of new features (Azure/azure-cli#16254, Azure/azure-cli#16257), we need more time investigating and writing tests.

[jiasli]

I have created a new PR #244 based on your commits so that you can still get credit and become a contributor. Please kindly see if the new PR satisfies your needs. Thanks.