Demo of how this should work (doesn't work yet)
Laid out the ideal -- rust-http not working
Compiles but example doesn't run
Set automatic 404 in defaulterrors

I wrongly assumed iron would set 404s when it finished `enter()`ing
the Stack. We need to set 404 ourselves if the status is not set.
Prints to console

Having yet started writing response to browser
Include enum variant in impl Show for ErrorResponse
Don't waste memory getting http status code
Error messages delivered to client
Added example using set_file()