Improve handling of downtime of an LDAP repository

This is one error that is thrown when this happens:

java.lang.NullPointerException
at com.atlassian.user.impl.ldap.security.authentication.DefaultLDAPAuthenticator.authenticate(DefaultLDAPAuthenticator.java:102)

Stack Trace: [hide]
java.lang.NullPointerException
at com.atlassian.user.impl.ldap.security.authentication.DefaultLDAPAuthenticator.authenticate(DefaultLDAPAuthenticator.java:102)
at com.atlassian.user.impl.delegation.security.authentication.DelegatingAuthenticator.authenticate(DelegatingAuthenticator.java:47)
at bucket.user.DefaultUserAccessor.authenticate(DefaultUserAccessor.java:742)
at sun.reflect.GeneratedMethodAccessor1002.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at org.springframework.aop.framework.AopProxyUtils.invokeJoinpointUsingReflection(AopProxyUtils.java:61)
at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:149)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:116)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:56)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:138)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:152)
at $Proxy56.authenticate(Unknown Source)
at com.atlassian.confluence.user.ConfluenceAuthenticator.authenticate(ConfluenceAuthenticator.java:43)
at com.atlassian.seraph.auth.DefaultAuthenticator.login(DefaultAuthenticator.java:93)
at com.atlassian.confluence.user.ConfluenceAuthenticator.login(ConfluenceAuthenticator.java:26)
at com.atlassian.seraph.filter.LoginFilter.doFilter(LoginFilter.java:125)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.johnson.filters.JohnsonFilter.doFilter(JohnsonFilter.java:96)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at org.springframework.orm.hibernate.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:170)
at bucket.custom.FlushingSpringSessionInViewFilter.doFilterInternal(FlushingSpringSessionInViewFilter.java:31)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:73)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:70)
at com.atlassian.util.profiling.filters.ProfilingFilter.doFilter(ProfilingFilter.java:132)
at com.atlassian.confluence.util.error.ProfilingAndErrorFilter.doFilter(ProfilingAndErrorFilter.java:27)

This is caused by:

javax.naming.ServiceUnavailableException: ldap.rave.ac.uk:389; socket closed; remaining name 'ou=people,dc=rave,dc=ac,dc=uk'
at com.sun.jndi.ldap.Connection.readReply(Connection.java:410)
at com.sun.jndi.ldap.LdapClient.getSearchReply(LdapClient.java:701)
at com.sun.jndi.ldap.LdapClient.search(LdapClient.java:624)
at com.sun.jndi.ldap.LdapCtx.doSearch(LdapCtx.java:1947)
at com.sun.jndi.ldap.LdapCtx.searchAux(LdapCtx.java:1809)
at com.sun.jndi.ldap.LdapCtx.c_search(LdapCtx.java:1734)
at com.sun.jndi.toolkit.ctx.ComponentDirContext.p_search(ComponentDirContext.java:368)
at com.sun.jndi.toolkit.ctx.PartialCompositeDirContext.search(PartialCompositeDirContext.java:328)
at com.sun.jndi.toolkit.ctx.PartialCompositeDirContext.search(PartialCompositeDirContext.java:313)
at com.atlassian.user.impl.ldap.search.DefaultLDAPUserAdaptor.search(DefaultLDAPUserAdaptor.java:80)
at com.atlassian.user.impl.ldap.search.DefaultLDAPUserAdaptor.getUserAttributes(DefaultLDAPUserAdaptor.java:124)
at com.atlassian.user.impl.ldap.search.DefaultLDAPUserAdaptor.getUserDN(DefaultLDAPUserAdaptor.java:143)