chore: improve error messages when connect failed

[terry-xiaoyu]

before:

(emqx@127.0.0.1)10> rp(sys:get_status(hd(L))).
{status,<0.2926.0>,
        {module,gen_server},
        [[{'$ancestors',[<0.2925.0>,<0.2924.0>,'172.100.239.2#7000',
                         eredis_cluster_pool,eredis_cluster_sup,<0.2623.0>]},
          {options,[]},
          {transport,gen_tcp},
          {'$initial_call',{eredis_client,init,1}}],
         running,<0.2925.0>,[],
         [{header,"Status for generic server <0.2926.0>"},
          {data,[{"Status",running},
                 {"Parent",<0.2925.0>},
                 {"Logged events",[]}]},
          {data,[{"State",
                  #state{host = "172.100.239.2",port = 7000,
                         password = <<"public">>,database = <<"0">>,
                         sentinel = undefined,reconnect_sleep = no_reconnect,
                         connect_timeout = 5000,socket = #Port<0.80>,
                         parser_state = #pstate{state = undefined,
                                                continuation_data = undefined},
                         queue = {[],[]}}}]}]]}
ok

after

(emqx@127.0.0.1)11> rp(sys:get_status(hd(L))).
{status,<0.2926.0>,
        {module,gen_server},
        [[{'$ancestors',[<0.2925.0>,<0.2924.0>,'172.100.239.2#7000',
                         eredis_cluster_pool,eredis_cluster_sup,<0.2623.0>]},
          {options,[]},
          {transport,gen_tcp},
          {'$initial_call',{eredis_client,init,1}}],
         running,<0.2925.0>,[],
         [{header,"Status for generic server <0.2926.0>"},
          {data,[{"Status",running},
                 {"Parent",<0.2925.0>},
                 {"Logged events",[]}]},
          {data,[{"State",
                  #state{host = "172.100.239.2",port = 7000,
                         password = "******",database = <<"0">>,sentinel = undefined,
                         reconnect_sleep = no_reconnect,connect_timeout = 5000,
                         socket = #Port<0.80>,
                         parser_state = #pstate{state = undefined,
                                                continuation_data = undefined},
                         queue = {[],[]}}}]}]]}