App Service/Function App - returning a helpful error when the name is in use

[tombuildsstuff]

The names used for Function Apps and App Services need to be globally unique. Currently when attempting to create either of these when the name is in use will return the (unclear/unhelpful) error:

Error: Error applying plan:

1 error(s) occurred:

* azurerm_function_app.test: 1 error(s) occurred:

* azurerm_function_app.test: web.AppsClient#CreateOrUpdate: Failure sending request: StatusCode=409 -- Original Error: failed request: autorest/azure: Service returned an error.Status=<nil> <nil>

This PR changes these to returning a friendlier error message if the Name specified is in use:

Error: Error applying plan:

1 error(s) occurred:

* azurerm_function_app.test: 1 error(s) occurred:

* azurerm_function_app.test: The name "test" used for the Function App needs to be globally unique and isn't available: Hostname 'test' already exists. Please select a different name.

and for an App Service:

Error: Error applying plan:

1 error(s) occurred:

* azurerm_app_service.test: 1 error(s) occurred:

* azurerm_app_service.test: The name "test" used for the App Service needs to be globally unique and isn't available: Hostname 'test' already exists. Please select a different name.

Fixes #994

[katbyte]

LGTM

[ghost]

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