RenderBody must be called from a Layout Page" provides no file information

[yishaigalatzer]

Repro:
Create new K Web project, comment out RenderBody() call in _Layout.cshtml:
Error message "InvalidOperationException: RenderBody must be called from a layout page."
Imagine a very large project with multiple layouts, perhaps very large files with lots of lines. Determining which one failed to call RenderBody() can be annoying. This error should list the file for which it's throwing the exception.

@MattGal

aspnet/Razor#280