Add in required error implementation.
Add in required error implementation.

This doesn't do anything, but at least docopt.rs can compile.

Fixes #9.

zsiciarz/docopt.rs,alexcrichton/docopt.rs,zsiciarz/docopt.rs,zsiciarz/docopt.rs,birkenfeld/docopt.rs,birkenfeld/docopt.rs,birkenfeld/docopt.rs,alexcrichton/docopt.rs,brandonson/docopt.rs,brandonson/docopt.rs,brandonson/docopt.rs