handle JNDI communication error properly

ActiveLDAP JNDI adapter doesn't handle JNDI communication error. i.e.:

 * It raises Java::JavaxNaming::CommunicationException
   (should be ActiveLdap::ConnectionError)
 * It does not trigger reconnecting

This patch fixes them.
(Tested with Ruby on Rails 3.2.12 and JRuby 1.7.4.)

I hope it helps.