In all cases where viewers are appointed by the courts of this commonwealth to view and lay out a public road, or where a review or re-review is ordered, the said viewers shall give the county commissioners or their clerk at least six days' notice in writing of the time and place of holding such view, review or re-review, and a copy of such written notice, properly attested, shall be filed with the report of such view, review or re-review, and a failure to comply with the provisions of the act shall be sufficient grounds to set aside the entire proceedings: Provided, That this act shall not apply to counties having special laws upon the subject.