add logging

Author: Davies Liu

core/src/main/java/org/apache/spark/memory/TaskMemoryManager.java

@@ -265,6 +265,7 @@ public MemoryBlock allocatePage(long size, MemoryConsumer consumer) {
     try {
       page = memoryManager.tungstenMemoryAllocator().allocate(acquired);
     } catch (OutOfMemoryError e) {
+      logger.warn("Failed to allocate a page ({} bytes), try again.", acquired);
       // there is no enough memory actually, it means the actual free memory is smaller than
       // MemoryManager thought, we should keep the acquired memory.
       acquiredButNotUsed += acquired;