Holidaymakers heading for ski resorts in France and Switzerland should brace themselves for bumps, bruises and broken bones on rock-hard slopes produced by snow cannon after a month without so much as a flake.  In the main destinations for British tourists, most of the pistes are open but this is often because they have been pumping artificial snow. It is feared that the conditions will lead to even more injuries than usual.  Patrick Guillaume, a mountain guide in alpe d'Huez, said: "There is not too much difference between artificial snow and natural snow except that the artificial kind is harder in the afternoon. Falls can be painful."  His warning was echoed by Robert Bolognesi, a spokesman for M?t?orisk, a Swiss weather consultancy. "The slopes are smooth and the stopping distance is greater. People are not always aware of this. You need to learn to control your speed," he said.  With no snow since early January in much of the alps and the Pyrenees, only resorts such as Les Deux alpes, which are at the highest altitudes, claim to be snow-covered. Others, such as Val d'Is?re, have no more than a sprinkling. Jean Marc Silva, director of France Montagnes, the French winter holiday tourist office, said: "It's tr