Refactor a bit.

service/rawdatastore/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 rawdatastore
+
+import (
+        "fmt"
+        "reflect"
+
+        "google.golang.org/appengine/datastore"
+)
+
+// These errors are returned by various rawdatastore.Interface methods.
+var (
+        ErrInvalidEntityType     = datastore.ErrInvalidEntityType
+        ErrInvalidKey            = datastore.ErrInvalidKey
+        ErrNoSuchEntity          = datastore.ErrNoSuchEntity
+        ErrConcurrentTransaction = datastore.ErrConcurrentTransaction
+        ErrQueryDone             = datastore.Done
+
+        // ErrMetaFieldUnset is returned from DSPropertyLoadSaver.{Get,Set}Meta
+        // implementations when the specified meta key isn't set on the struct at
+        // all.
+        ErrMetaFieldUnset = fmt.Errorf("gae: meta field unset")
+)
+
+// ErrFieldMismatch is returned when a field is to be loaded into a different
+// type than the one it was stored from, or when a field is missing or
+// unexported in the destination struct.
+// StructType is the type of the struct pointed to by the destination argument
+// passed to Get or to Iterator.Next.
+type ErrFieldMismatch struct {
+        StructType reflect.Type
+        FieldName  string
+        Reason     string
+}
+
+func (e *ErrFieldMismatch) Error() string {
+        return fmt.Sprintf("gae: cannot load field %q into a %q: %s",
+                e.FieldName, e.StructType, e.Reason)
+}