Why do some artists spend their whole careers doing stupid stuff like mutilating mannequins or painting old bits of wood with baffling phrases? Why does everyone in the art world get paid, apart from the artists? Why do most art students spend years doing their MA, closely followed by them doing sweet FA? Who are the HoWiAs, and what the hell do they think they're doing? Artist and writer Alistair Gentry reveals all...it's career suicide