AspNetCoreModuleV2 report App_Offline.htm

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

When deployment fails, _App_Offline.htm_ is kept in the application directory and must be manually removed. This is [documented](https://learn.microsoft.com/en-us/iis/publish/deploying-application-packages/taking-an-application-offline-before-publishing) but extremely difficult to diagnose. The web application just serves blank 503 page with no indication of the reason. Failed request tracing does not help:


### Describe the solution you'd like

Indicate somewhere that the presence of _App_Offline.htm_ is the reason for the 503 response. This could be a substatus code, error code, failed request tracing message and/or a response header or content.

### Additional context

_No response_