Phoebe Gill

As an interested third party, is this PR likely to get merged at any point in the near future?

As an added bonus, switching to `error-chain` will ensure that all our errors implement `Send`, which will make parallelisation (https://github.com/killercup/cargo-edit/issues/158) quite a bit easier.

I think I've run into a related issue in [cargo-edit](https://github.com/killercup/cargo-edit). Our usage string is: ``` Usage: cargo add [--dev|--build|--optional] [--vers=|--git=|--path=] [options] cargo add ... [--dev|--build|--optional] [options] cargo add (-h|--help) cargo...

Interesting. It turns out it catches other problems as well. The build's just about to fail. Which nicely proves that this linter does successfully fail the build if it detects...

OK. I've raised https://github.com/alphagov/tech-docs-template/pull/187 to implement.