[BugBash][CLI]no error reported if specify un-supported unit count while update

[juniwang]

Describe the bug

[Cli]cannot update unit count using cli

To Reproduce

• Open Cloud Shell on Azure portal
• Create a new Web PubSub: az webpubsub create -g <groupname> -n <name> --sku Standard_S1
• Update unit count by az webpubsub update -g <groupname> -n <name> --unit-count 3
• The command Succeeded immediately. But if run ``az webpubsub show -g -n , the result shows its unit count is still 1`.

Exceptions (if any)

The command should report an error saying that 3 is not a valid value for unit count

Further technical details

• SDK and its version you use

[zackliu]

https://github.com/Azure/azure-cli-extensions/pull/4049