return 404 for matchups not found for PUT and DELETE

Expose less information to the public when a `Matchup` for a given
`id` is not found. Mongoid will raise an exception by default when
a document is not found using the `find` method.

We are choosing to keep the default settings for `Mongoid.raise_not_found_error`
to `true`. In response, we use the `where` query method specifying
`id`, which returns `nil` if there are no matching documents. Otherwise,
we return a `404`.