Added extra reason for a Pooled Connection to be closed

If there is an error then the underlying connection is closed but that
was being incorrectly logged.

Author: rozza

driver-core/src/main/com/mongodb/connection/DefaultConnectionPool.java

@@ -483,6 +483,8 @@ public void close(final UsageTrackingInternalConnection connection) {
                 reason = "it is past its maximum allowed life time";
             } else if (pastMaxIdleTime(connection)) {
                 reason = "it is past its maximum allowed idle time";
+            } else if (connection.isClosed()) {
+                reason = "the underlying connection was closed";
             } else {
                 reason = "the pool has been closed";
             }