logging: Add subsystem to log API (#19002)

Create new code paths for multiple subsystems in the code. This will
make maintaing this easier later.

Also introduce bugLogIf() for errors that should not happen in the first
place.

cmd/bucket-policy.go

@@ -53,7 +53,7 @@ func (sys *PolicySys) IsAllowed(args policy.BucketPolicyArgs) bool {
 
 	// Log unhandled errors.
 	if _, ok := err.(BucketPolicyNotFound); !ok {
-		logger.LogIf(GlobalContext, err)
+		internalLogIf(GlobalContext, err, logger.WarningKind)
 	}
 
 	// As policy is not available for given bucket name, returns IsOwner i.e.