Map result values instead to use a match block

Summary:
To map a result, ie a success T or an error E, into another result,
ie a success U or an error F means the code focus on T -> U, without
regard it's an Ok, and E -> F, without regard it's an Err value.

As such, it's seems more natural to express the idea with
methods manipulating tbe values than to recreate the enum container too.

The Serde library uses those map and map_err methods.

Test Plan:

#[get("/player/<name>")]
pub fn get_player(connection: DatabaseConnection, name: String) -> ApiJsonResponse<Player> {
    players
        .filter(username.eq(&name))
        .first::<Player>(&*connection)
        .into_json_response()
}

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1950