Adding error text for nothingSchema validation

[Cyri-L]

before:

var NothingSchema = module.exports = Schema.patterns.NothingSchema = Schema.extend({
    errors: function (instance) {
        return false
    },

after:

var NothingSchema = module.exports = Schema.patterns.NothingSchema = Schema.extend({
    errors: function (instance) {
        if(instance != null)
            return "this key is not allowed in strict mode";
        return false
    },

[YouriT]

ping @molnarg