Provide fallback for people who haven't installed npm modules.

[nataliecarey]

To test this:

npx govuk-prototype-kit create --version=github:alphagov/govuk-prototype-kit#install-on-dev-failure example-kit
cd example-kit
rm -Rf node_modules
npm run dev

This emulates one person creating it, pushing it (with node_modules ignored as it should be), someone else cloning it and not realising they should install node modules before starting.