more data to exception msgs

server/src/main/java/com/metamx/druid/coordination/ServerManager.java

@@ -196,7 +196,10 @@ public class ServerManager implements QuerySegmentWalker
           oldQueryable.close();
         }
         catch (IOException e) {
-          log.makeAlert(e, "Unable to close segment %s", segment.getIdentifier()).emit();
+          log.makeAlert(e, "Exception closing segment")
+             .addData("dataSource", dataSource)
+             .addData("segmentId", segment.getIdentifier())
+             .emit();
         }
       } else {
         log.info(

server/src/test/java/com/metamx/druid/index/ReferenceCountingSegmentTest.java

@@ -129,6 +129,8 @@ public class ReferenceCountingSegmentTest
     segment.increment();
     segment.increment();
     segment.increment();
+    Assert.assertTrue(segment.getNumReferences() == 0);
+
     segment.close();
     Assert.assertTrue(segment.getNumReferences() == 0);
   }