Add debug options to LDAP for diagnostic purposes

[TheWitness]

Describe the bug

When attempting to login with LDAP, backtrace errors are being generated.

AUTH LDAP: Protocol Error, unable to start TLS communications (Can't contact LDAP server) on Server (192.168.11.105)
ERROR PHP NOTICE: Undefined index: error_num in file: /var/www/html/cacti/lib/ldap.php  on line: 177
ERROR PHP NOTICE: Undefined index: error_num in file: /var/www/html/cacti/auth_login.php  on line: 160

Seems related to recent LDAP changes in error handling.

Expected behavior

LDAP, even with configuration issues should not generate backtraces.

[TheWitness]

Added 'debug' mode to LDAP. Fixed the error now.