Add error messages from HttpCompileError to the error response in debug ...

This is a change to inspect HttpCompileException (other exception types later?) in order to add error details to the response DTO in debug mode. Without this, it's difficult to determine what the errors are in your Razor pages. Re-using ResponseError for this produces suitable output on the service snapshot page.

A future enhancement could be to display the snippet of source as well, but this alone saves a lot of debugging headache.