This checks if the next middleware to be called is nil, and if so returns
ServerFailure and an error. This makes the next calling more robust and
saves some lines of code.
Also prefix the error with the name of the middleware to aid in
debugging.
Add a new middleware that tells you who you are; IP, port and transport
is echoed back.
Also some various cleanup and documentation improvements while at it:
* ResponseWriter: improve the documentation of these helper functions.
* And add an NextHandler for use in tests. Make chaos_test.go and
* whoam_test.go use it.
