User's key should satisfy the requirement of `mc config host add`.
Check access key and secret key length before adding a new user,
avoid creating a useless user which cannot be added into config
host or log into the browser.
This is an effort to remove panic from the source.
Add a new call called CriticialIf, that calls LogIf and exits.
Replace panics with one of CriticalIf, FatalIf and a return of error.
