Move multierror logic into luci-go.

common/errors/errors.go

-// Copyright 2015 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-package errors
-
-import (
-        "fmt"
-        "path/filepath"
-        "runtime"
-)
-
-// 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 {
-        // Orig contains the original data (error or otherwise) which was passed to
-        // the marker function.
-        Orig interface{}
-
-        msgFmt string
-}
-
-func (te *MarkedError) Error() string {
-        return fmt.Sprintf(te.msgFmt, te.Orig)
-}
-
-// MakeMarkFn returns a new 'marker' function for your library. The name parameter
-// should probably match your package name, but it can by any string which
-// will help identify the error as originating from your package.
-func MakeMarkFn(name string) func(interface{}) error {
-        fmtstring1 := name + " - %s:%d - %%+v"
-        fmtstring2 := name + " - %+v"
-        return func(errish interface{}) error {
-                if errish == nil {
-                        return nil
-                }
-                _, filename, line, gotInfo := runtime.Caller(1)
-                pfx := fmtstring2
-                if gotInfo {
-                        pfx = fmt.Sprintf(fmtstring1, filepath.Base(filename), line)
-                }
-                return &MarkedError{errish, pfx}
-        }
-}