Log a warn before exiting when an illegal access not allowed is done on FileDescriptor#fd (#2749)

diff --git a/bookkeeper-server/src/main/java/org/apache/bookkeeper/util/NativeIO.java b/bookkeeper-server/src/main/java/org/apache/bookkeeper/util/NativeIO.java
index 6f0bdf1..9fea0c6 100644
--- a/bookkeeper-server/src/main/java/org/apache/bookkeeper/util/NativeIO.java
+++ b/bookkeeper-server/src/main/java/org/apache/bookkeeper/util/NativeIO.java
@@ -65,8 +65,8 @@
         } catch (Exception e) {
             // We don't really expect this so throw an assertion to
             // catch this during development
-            assert false;
             LOG.warn("Unable to read {} field from {}", fieldName, cls.getName());
+            assert false;
         }
 
         return field;