make deploy: Do `git pull --rebase` on gh-pages

In case somebody else has pushed changes to the gh-pages branch.
make deploy: Remove previous `static` directory

In case a previous `make deploy` had failed before the `trap` command
had been run.
Makefile: Enable pandoc "auto_identifiers" extension

So that each header gets an html ID attached to it.
CSS: Remove border around stb-tester logo in header in runner.html
precondition: Explain the difference between errors & failures

This is thanks to Will's feedback.