changed virtual buffer cache to throw an error if opened (closed) while open (closed)
diff --git a/pregelix-dataflow/src/main/java/edu/uci/ics/pregelix/dataflow/context/RuntimeContext.java b/pregelix-dataflow/src/main/java/edu/uci/ics/pregelix/dataflow/context/RuntimeContext.java
index 8d6ab38..0f4afd3 100644
--- a/pregelix-dataflow/src/main/java/edu/uci/ics/pregelix/dataflow/context/RuntimeContext.java
+++ b/pregelix-dataflow/src/main/java/edu/uci/ics/pregelix/dataflow/context/RuntimeContext.java
@@ -75,7 +75,7 @@
         resourceIdFactory = new ResourceIdFactory(0);
     }
 
-    public void close() {
+    public void close() throws HyracksDataException {
         for (Entry<Long, List<FileReference>> entry : iterationToFiles.entrySet())
             for (FileReference fileRef : entry.getValue())
                 fileRef.delete();