Nailgun validators shouldn't mutate data in any way

Right now there is one big issue with our validators - they do thing they are not supposed to - converting input data from JSON string to dict. They will do it anyway on the inside, but we shouldn't return the result of their checks as a Python dict, actually, they shouldn't return anything, just raise exceptions. So, we need to rewrite handlers to serialize input data by themselves after processing it through validator, catching errors and raising corresponding exceptions if something is wrong with data.