Exercises should lead up to the capstone

[stellentus]

An exercise (or five!) should involve working with byte slices, readers, and writers. Maybe even posting HTTP.

(Also, I think it should be possible to avoid mentioning the header in the explanation. We really only care about the body. It's helpful to understand an HTTP header, but for students who are already getting overwhelmed, the header is something that can be ignored.)

[grrtrr]

Perhaps this could involve the error exercise(s) as well. I did not really like the one I used DivideErr, since it was fairly contrived and non-practical. Could deal with temporary (503) or fatal (404) Http responses instead.

[stellentus]

For errors, I'd suggest an exercise which involves cascading errors up to the main function, and exiting the program if there's an error. I know that's extremely simple, but that's 99% of what a beginner needs to do with errors.