validationMessage (rule, validationContext, vm) {
    const generators = this.options.locales[this.getLocale(vm)]
    if (generators.hasOwnProperty(rule)) {
      return generators[rule](validationContext)
    }
    if (generators.hasOwnProperty('default')) {
      return generators.default(validationContext)
    }
    return 'Invalid field value'
  }
Get the validation message for a particular error.