Consistent error handling

[andreasohlund]

As part of doing #41 the question of error handling came up. In this case if the user is providing a bad nuget id on the url we just throw new Exception. How should we play this?

Seems like a combo of custom exception like mentioned here

http://bytefish.de/blog/consistent_error_handling_with_nancy/

Combined with how octopus is doing their custom error pages? http://paulstovell.com/blog/consistent-error-handling-with-nancy

Thoughts?