No error validation prompts when creating a new web app with an existing name

[v-mengwe]

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OS: Win10 Build Version: 20220225.1 Regression: Previous Release

Repro Steps:

1. F1 -> Azure App Service: Create New Web App... -> Enter an existing name.
2. Check whether an error validation prompts.

Expect: An error validation prompts.

Actual: No error validation prompts.

[nturinski]

This will be fixed by updating the @microsoft-vscode-azext-appservice extension