Move Err() method to Iterator interface, fix fallout

graph/iterator.go

@@ -100,6 +100,9 @@ type Iterator interface {
 	// Contains returns whether the value is within the set held by the iterator.
 	Contains(Value) bool
 
+	// Err returns the error (if any) encountered during iteration.
+	Err() error
+
 	// Start iteration from the beginning
 	Reset()
 
@@ -160,9 +163,6 @@ type Nexter interface {
 	// between the two cases.
 	Next() bool
 
-	// Err returns the error (if any) encountered during iteration.
-	Err() error
-
 	Iterator
 }
 

graph/iterator/optional_iterator.go

@@ -76,6 +76,10 @@ func (it *Optional) ResultTree() *graph.ResultTree {
 	return graph.NewResultTree(it.Result())
 }
 
+func (it *Optional) Err() error {
+	return it.subIt.Err()
+}
+
 func (it *Optional) Result() graph.Value {
 	return it.result
 }