getById: returns a GraphQL friendly throwable instead of TDBMException

[gulien]

Currently, the method getById of a Dao throws a TDBMException when an error occurs.

It would be better to have a GraphQL friend exception when the entity does not exist for given ID (with a 404 HTTP code).

Example:

    /**
     * @Query
     */
    public function getUserById(User $user): User
    {
        return $user;
    }