Refactor a bit.

service/rawdatastore/errors.go

Index: service/rawdatastore/errors.go
diff --git a/service/rawdatastore/errors.go b/service/rawdatastore/errors.go
new file mode 100644
index 0000000000000000000000000000000000000000..7ba1af9a406241630de7b8b6a84dafd95300183f
--- /dev/null
+++ b/service/rawdatastore/errors.go
@@ -0,0 +1,42 @@
+// 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)
+}