Sitelets.HttpModule should throw an error if no Sitelet definition is found

[Jand42]

Now that the location of the Sitelet definition is stored in WebSharper metadata to speed up site initialization, the project containing the definition must have the WebSharper.FSharp/CSharp package reference, even it is server-side only. Missing this, Sitelet runtime finds nothing to run and currently the Sitelets.HttpModule just installs an empty Sitelet: a handler that handles nothing.

This causes projects to break silently when updating from WS 4.2 or lower.

Throwing an error instead of defaulting to empty Sitelet would be better. If a project does not use Sitelets, it should not set the Sitelets.HttpModule handler in web.config / ASP.NET Core startup.