differenciate 'SHOULD' (warning) from 'MUST' (error)

[liangchenye]

It is the successor of #324.
We can wrap all the 'SHOULD' errors into one type(similar implementation of 'os.PathError') and then checks the returned type of errors with the log-level. If log-level is 'warning', we shoud also return the 'SHOULD testing' result.

[liangchenye]

checked in. close