Add x-ms-error-response for 404 errors in Compute

The error code of 404 lack the swagger extension of x-ms-error-response which cause SDK generation problem.
https://github.com/Azure/azure-rest-api-specs/blob/main/specification/web/resource-manager/Microsoft.Web/stable/2021-01-01/AppServicePlans.json#L140

http://azure.github.io/autorest/extensions/#x-ms-error-response

Please add the extension into the Swagger.

Open specification/web/resource-manager/Microsoft.Web/stable/2021-01-01/AppServicePlans.json at the referenced line and read the x-ms-error-response documentation. Check the 404 response definition in Compute, add the requested extension, and verify that the Swagger contains it for the affected response.

Written by the indexing model from the issue text.