When Spark returns 404 ("The requested route has not been mapped in Spark"), I'd like it to tell me the request URL that failed. 

Can be handy sometimes. E.g: I'm using Spark as a stub for some backend REST services when developing a web client for those services. Not always sure of what my client is doing (hey, nobody's perfect right ;)), so this small addition helps me a lot. 

This may be useful for others, hence the pull request.