Fix error message from KeyForObj when passing an invalid struct.

service/datastore/properties.go

Index: service/datastore/properties.go
diff --git a/service/datastore/properties.go b/service/datastore/properties.go
index 4267d029ad7c7387073cfb2a70e5c369aec6628c..95a4e6069267aa43061d99b2a906bc39a2713619 100644
--- a/service/datastore/properties.go
+++ b/service/datastore/properties.go
@@ -649,19 +649,6 @@ type MetaGetter interface {
 	//     // BadFlag  Toggle `gae:"$flag3"` // ILLEGAL
 	//   }
 	GetMeta(key string) (interface{}, error)
-
-	// GetMetaDefault is GetMeta, but with a default.
-	//
-	// If the metadata key is not available, or its type doesn't equal the
-	// homogenized type of dflt, then dflt will be returned.
-	//
-	// Type homogenization:
-	//   signed integer types -> int64
-	//   bool                 -> Toggle fields (bool)
-	//
-	// Example:
-	//   pls.GetMetaDefault("foo", 100).(int64)
-	GetMetaDefault(key string, dflt interface{}) interface{}
 }
 
 // PropertyLoadSaver may be implemented by a user type, and Interface will
@@ -679,11 +666,6 @@ type PropertyLoadSaver interface {
 	// then the PropertyMap contains all the metadata (e.g. '$meta' fields)
 	// which was held by this PropertyLoadSaver.
 	Save(withMeta bool) (PropertyMap, error)
-
-	// Problem indicates that this PLS has a fatal problem. Usually this is
-	// set when the underlying struct has recursion, invalid field types, nested
-	// slices, etc.
-	Problem() error
 }
 
 // MetaGetterSetter is the subset of PropertyLoadSaver which pertains to
@@ -787,11 +769,6 @@ func (pm PropertyMap) GetAllMeta() PropertyMap {
 	return ret
 }
 
-// GetMetaDefault is the implementation of PropertyLoadSaver.GetMetaDefault.
-func (pm PropertyMap) GetMetaDefault(key string, dflt interface{}) interface{} {
-	return GetMetaDefaultImpl(pm.GetMeta, key, dflt)
-}
-
 // SetMeta implements PropertyLoadSaver.SetMeta. It will only return an error
 // if `val` has an invalid type (e.g. not one supported by Property).
 func (pm PropertyMap) SetMeta(key string, val interface{}) error {
@@ -833,18 +810,28 @@ func isMetaKey(k string) bool {
 	return k == "" || k[0] == '$'
 }
 
-// GetMetaDefaultImpl is the implementation of PropertyLoadSaver.GetMetaDefault.
+// GetMetaDefault is a helper for GetMeta, allowing a default value.
+//
+// If the metadata key is not available, or its type doesn't equal the
+// homogenized type of dflt, then dflt will be returned.
 //
-// It takes the normal GetMeta function, the key and the default, and returns
-// the value according to PropertyLoadSaver.GetMetaDefault.
-func GetMetaDefaultImpl(gm func(string) (interface{}, error), key string, dflt interface{}) interface{} {
+// Type homogenization:
+//   signed integer types -> int64
+//   bool                 -> Toggle fields (bool)
+//
+// Example:
+//   pls.GetMetaDefault("foo", 100).(int64)
+func GetMetaDefault(getter MetaGetter, key string, dflt interface{}) (interface{}, error) {
 	dflt = UpconvertUnderlyingType(dflt)
-	cur, err := gm(key)
+	cur, err := getter.GetMeta(key)
 	if err != nil {
-		return dflt
+		if err == ErrMetaFieldUnset {
+			return dflt, nil
+		}
+		return nil, err
 	}
 	if dflt != nil && reflect.TypeOf(cur) != reflect.TypeOf(dflt) {
-		return dflt
+		return dflt, nil
 	}
-	return cur
+	return cur, nil
 }