How the artist foreshadowed our compulsive romance with technology. "The invention of Andy Warhol was among his earliest works, a forerunner of the alchemical process by which he turned the humble Campbell’s soup can into an icon. Recognisable meant replicable, which is how a nobody becomes a star." Beneath his stylish veneer, there was a loneliness that prefigures our age of social media. Olivia Laing explores.