{ARM} ts: show command bug fixed, improved error handling for export

[detienne20]

Description

This PR addresses two customer reported issues.

For the first (#16736) the error handling has been improved by throwing an incorrect usage error when version is not specified when calling ts export.

Testing: 
 
 az ts export -g resourceGroup --name templatespec name --output-folder
 az ts export -s templatespecID 
 
 Both commands should fail with " a template spec version must be specified for 
 export."

For the second ( #16801 (comment)) az ts show now reports existing template spec versions instead of returning null.

Testing:

  1. Created a template spec and template spec version(s). 
 
  2. Retrieve a template spec  via:
      ts show --template-spec resourceID 
      ts show -g resourceGroup --name TemplateSpecName 

  3. Returned object should have the name of the template spec versions in the versions 
     property. 

[yonzhan]

ARM

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