[Kerberos] Add debug log statement for exceptions (#32663)

This commit adds missing debug log statements for exceptions
that occur during ticket validation. I thought these
get logged somewhere else in authentication chain
but even after enabling trace logs I could not see them
logged. As the Kerberos exception messages are cryptic
adding full stack trace would help debugging faster.

Author: bizybot

x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/kerberos/KerberosRealm.java

@@ -179,12 +179,15 @@ protected String maybeRemoveRealmName(final String principalName) {
 
     private void handleException(Exception e, final ActionListener<AuthenticationResult> listener) {
         if (e instanceof LoginException) {
+            logger.debug("failed to authenticate user, service login failure", e);
             listener.onResponse(AuthenticationResult.terminate("failed to authenticate user, service login failure",
                     unauthorized(e.getLocalizedMessage(), e)));
         } else if (e instanceof GSSException) {
+            logger.debug("failed to authenticate user, gss context negotiation failure", e);
             listener.onResponse(AuthenticationResult.terminate("failed to authenticate user, gss context negotiation failure",
                     unauthorized(e.getLocalizedMessage(), e)));
         } else {
+            logger.debug("failed to authenticate user", e);
             listener.onFailure(e);
         }
     }