A Gardnerville family is mourning the loss a 4-year-old boy who died Sunday afternoon after being hit by a runaway U-Haul truck in a relative's driveway near Spring Valley. Four-year-old Mason Geist was transported from the scene of the 3:45 p.m. accident to Carson Valley Medical Center, where he was pronounced dead shortly after arrival."He was a kick in the pants. He loved Batman," relative Chad Lathrop said. "It's an awful situation. The kid was his life."Lathrop was referring to Gardnerville Ranchos resident and stone and tile contractor Robert Geist, who, along with Mason, was helping Lathrop move out of …