client: add examples, short flags and better error handling

[fmuyassarov]

Improve user experience.

• add usage examples to export and compat commands
• add -p short flag for --path option
• silence usage output on runtime errors for cleaner error messages

Examples

1. Usage silencing (before)

   $ ./bin/nfd export features --pathd
   Error: unknown flag: --pathd
   Usage:
     nfd export features [flags]
   
   Flags:
     -h, --help          help for features
         --path string   export to this JSON path
   
   unknown flag: --pathd
   

   after

   $ ./bin/nfd export features --pathd
   Error: unknown flag: --pathd
   unknown flag: --pathd
   

   before

   $  ./bin/nfd  compat validate-node --output-jsond true
   Error: unknown flag: --output-jsond
   Usage:
     nfd compat validate-node [flags]
   
   Flags:
     -h, --help                       help for validate-node
         --image string               the URL of the image containing compatibility metadata
         --output-json                print a JSON object
         --plain-http                 use of HTTP protocol for all registry communications
         --platform string            the artifact platform in the format os[/arch][/variant][:os_version]
         --registry-password-stdin    read registry password from stdin
         --registry-token-stdin       read registry access token from stdin
         --registry-username string   registry username
         --tags strings               a list of tags that must match the tags set on the compatibility objects
   
   unknown flag: --output-jsond
   

   after

   $ ./bin/nfd  compat validate-node --output-jsond true
   Error: unknown flag: --output-jsond
   unknown flag: --output-jsond
   

2. Helper examples:

   $ ./bin/nfd export features -h
   Export features for given node
   
   Usage:
     nfd export features [flags]
   
   Examples:
   
   # Export node features to stdout (prints to terminal)
   nfd export features
   
   # Export node features to a file instead
   nfd export features --path /tmp/features.json
   
   Flags:
     -h, --help          help for features
     -p, --path string   export to this JSON path
   

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[marquiz]

Thanks @fmuyassarov for the enhancement, looks good to me

@mfranczy @ChaoyiHuang @vsoch

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fmuyassarov, marquiz

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