Adds display error for scheduler

[TimothyClayton]

WIP
Displays error for patchTimeSlot calls (TimeSlotsController#update)

Testing Notes:

To force error on drag/drop, replace the update action in staff/grids/time_slots_controller with the below:

  def update
    @time_slot.errors.add(:conference_day, "is unavailable")
    @time_slot.errors.add(:start_time, "is too early in the morning")

    respond_to do |format|
      format.json { render json: update_response.merge({ errors: @time_slot.errors.full_messages, status: :bad_request }) }
    end
  rescue StandardError => e
    render json: update_response.merge({errors: ["There was a problem updating this time slot."], status: 500})
  end

[TimothyClayton]

For whatever reason, this pull request was not registering my commits.