404 if the Repo can't find a document

With the introduction of a Repo for fetching documents, the Repo would
throw an error if params[:section_id] didn't match a placeholder file
instead of allowing the controller to respond with a 404. This commit
checks for the existence of a file first, which if false allows the
controller to respond with a 404.
Add cucumber to manuals-frontend.

Gem versions are pessimistically fixed to match those in other projects
(rspec 2 rather than 3, for example).

Also include two passing tests covering local manual and section
browsing and the fix introduced in 465ef06.
Update language used to match the rest of the spec
Merge pull request #48 from alphagov/fetch-with-repo-can-now-404

404 if the Repo can't find a document