Error: An understood failure mode that indicates the request was flawed (duplicate device name)
Fault: a failure mode where the request is valid but application failed anyway
Error对应4xx，表示client或者input有问题
Fault对应5xx，表示service或者output有问题
一般而言，应该分类，alarm只应该对fault，有时候不确定时候就先保守地归类为Fault，后面转为Error。
