A special exception is a use or structure that may not be appropriate generally or without restriction throughout a district but which, if controlled as to number, area, location or relation to neighborhood, would promote the public health, safety, welfare, comfort, convenience or the general welfare. Such uses or structures may be permissible in a zoning district as a special exception only if specific provision for such use or structure is made in the district. A special exception shall not be issued for any other use or structure.