Show helpful warning or error message when failed to login with LDAP user account

Reproduce Steps:
1. enable LDAP authentication according to http://www.jspwiki.org/wiki/WebContainerAuthenticationViaLDAP
2. login JSPWiki with LDAP user account which doesn't exist on LDAP server
3. login JSPWiki with LDAP user account which exists on LDAP server but the LDAP server is not started

Expected Result:
There is some message to show the background problem to help people out of trouble

Actual Result:
No any message displays there, which put user in puzzle.