Better attempt at an appengine wrapper.

go/src/infra/gae/libs/wrapper/gae/commonErrors/errors.go

Index: go/src/infra/gae/libs/wrapper/gae/commonErrors/errors.go
diff --git a/go/src/infra/gae/libs/wrapper/gae/commonErrors/errors.go b/go/src/infra/gae/libs/wrapper/gae/commonErrors/errors.go
new file mode 100644
index 0000000000000000000000000000000000000000..780787e37e15b094475f9b9e82bd9facfcca1c09
--- /dev/null
+++ b/go/src/infra/gae/libs/wrapper/gae/commonErrors/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 commonErrors
+
+import (
+	"reflect"
+
+	"appengine/datastore"
+	"appengine/memcache"
+	"appengine/taskqueue"
+)
+
+// Common errors originating from the GAE SDK
+var (
+	ErrCASConflictMC = memcache.ErrCASConflict
+	ErrNoStatsMC     = memcache.ErrNoStats
+	ErrCacheMissMC   = memcache.ErrCacheMiss
+	ErrNotStoredMC   = memcache.ErrNotStored
+	ErrServerErrorMC = memcache.ErrServerError
+
+	ErrInvalidKeyDS            = datastore.ErrInvalidKey
+	ErrConcurrentTransactionDS = datastore.ErrConcurrentTransaction
+	ErrNoSuchEntityDS          = datastore.ErrNoSuchEntity
+	ErrInvalidEntityTypeDS     = datastore.ErrInvalidEntityType
+
+	ErrTaskAlreadyAddedTQ = taskqueue.ErrTaskAlreadyAdded
+)
+
+// TODO(riannucci): Add common internal errors which originate from the
+// dev_appserver SDK
+
+// ErrFieldMismatchDS returns a datastore.ErrFieldMismatch struct filled with
+// the given specification
+func ErrFieldMismatchDS(structType reflect.Type, fieldName, reason string) error {
+	return &datastore.ErrFieldMismatch{
+		StructType: structType,
+		FieldName:  fieldName,
+		Reason:     reason,
+	}
+}