Fix error reporting for invalid routes.

Due to the way the `[]=` method is implemented in ActiveModel errors[1],
this was setting the errors to an array containing another array. Using
`set` does the correct thing.

[1] http://apidock.com/rails/v4.0.2/ActiveModel/Errors/%5B%5D%3D
Don't register routes for a placeholder item.

This is to allow content-store to contain minimal details for items that
are still handled in other ways.  This is needed to allow for tagging
content-items with these among other things...
Update docs to mention placeholder items
Merge pull request #50 from alphagov/placeholder_items

 Don't register routes for a placeholder item.