Move multierror logic into luci-go.

common/errors/markederror.go

Index: common/errors/markederror.go
diff --git a/common/errors/errors.go b/common/errors/markederror.go
similarity index 92%
rename from common/errors/errors.go
rename to common/errors/markederror.go
index 3deb920326171543e6b5bda57141516d01e60c5b..ae4e885824dc1a0337a5991f559a2974e676aa2d 100644
--- a/common/errors/errors.go
+++ b/common/errors/markederror.go
@@ -5,11 +5,15 @@
 package errors
 
 import (
+	"errors"
 	"fmt"
 	"path/filepath"
 	"runtime"
 )
 
+// New is a pass-through version of the standard errors.New function.
+var New = errors.New
+
 // MarkedError is the specific error type retuned by MakeMarkFn. It's designed
 // so that you can access the underlying object if needed.
 type MarkedError struct {