Show error page or error JSON, depending on Accept header.

Error page is empty instead of rendering the view currently due to
Nancy bug 1718 (https://github.com/NancyFx/Nancy/issues/1718).
Wait for SIGINT or SIGTERM instead of enter on unix systems.
Install SlowCheetah for transforming web app.config.
.NET 4.5.1 -> .NET 4.5 for everything, for mono support.