Remove graph.Err helper function

Since Err() is now a member of the Iterator interface, we don't need
this helper function anymore.

graph/iterator/and_iterator.go

@@ -154,7 +154,7 @@ func (it *And) Next() bool {
 			return graph.NextLogOut(it, curr, true)
 		}
 	}
-	if err := graph.Err(it.primaryIt); err != nil {
+	if err := it.primaryIt.Err(); err != nil {
 		it.err = err
 	}
 	return graph.NextLogOut(it, nil, false)

graph/iterator/hasa_iterator.go

@@ -203,7 +203,7 @@ func (it *HasA) Next() bool {
 	it.resultIt = &Null{}
 
 	if !graph.Next(it.primaryIt) {
-		if err := graph.Err(it.primaryIt); err != nil {
+		if err := it.primaryIt.Err(); err != nil {
 			it.err = err
 		}
 		return graph.NextLogOut(it, 0, false)

graph/iterator/linksto_iterator.go

@@ -166,7 +166,7 @@ func (it *LinksTo) Next() bool {
 	}
 
 	// If there's an error in the 'next' iterator, we save it and we're done.
-	if err := graph.Err(it.nextIt); err != nil {
+	if err := it.nextIt.Err(); err != nil {
 		it.err = err
 		return false
 	}

graph/iterator/materialize_iterator.go

@@ -206,7 +206,7 @@ func (it *Materialize) Next() bool {
 	}
 	if it.aborted {
 		n := graph.Next(it.subIt)
-		if err := graph.Err(it.subIt); err != nil {
+		if err := it.subIt.Err(); err != nil {
 			it.err = err
 		}
 		return n
@@ -296,7 +296,7 @@ func (it *Materialize) materializeSet() {
 			it.actualSize += 1
 		}
 	}
-	if err := graph.Err(it.subIt); err != nil {
+	if err := it.subIt.Err(); err != nil {
 		it.err = err
 	} else if it.aborted {
 		if glog.V(2) {

graph/iterator/not_iterator.go

@@ -88,7 +88,7 @@ func (it *Not) Next() bool {
 			return graph.NextLogOut(it, curr, true)
 		}
 	}
-	if err := graph.Err(it.allIt); err != nil {
+	if err := it.allIt.Err(); err != nil {
 		it.err = err
 	}
 	return graph.NextLogOut(it, nil, false)

graph/iterator/or_iterator.go

@@ -148,7 +148,7 @@ func (it *Or) Next() bool {
 			return graph.NextLogOut(it, it.result, true)
 		}
 
-		if err := graph.Err(curIt); err != nil {
+		if err := curIt.Err(); err != nil {
 			it.err = err
 			return graph.NextLogOut(it, nil, false)
 		}

graph/iterator/value_comparison_iterator.go

@@ -134,7 +134,7 @@ func (it *Comparison) Next() bool {
 			return true
 		}
 	}
-	if err := graph.Err(it.subIt); err != nil {
+	if err := it.subIt.Err(); err != nil {
 		it.err = err
 	}
 	return false