Add custom 403 and 404 pages

[tusmester]

Currently there is no custom 403/404 page in this component. Simply moving the old ones from TFS is not enough, because they contain info text content with the old demo content type WebContentDemo - that should be converted to something that does not have "demo" in its name...

• Task1: convert info content in the TFS/... Default_Site/error_pages folder to a different content type and move them to this repo (error_pages.Content too, because it contains additional permission entries for visitors!)
• Task2: add the system.webServer.httpErrors section from web.config in TFS to the transform file here.