Address some replication test failures due to cluster race conditions

Change some of the helper functions in the `repl_util` module to handle errors more sensibly so that cluster setup race conditions do not cause unnecessary test failures.

This should address riak_test failures such as this: 

```
07:29:23.446 [error] 
================ replication2_upgrade failure stack trace =====================
{function_clause,
    [{rt,wait_until,
         [{badrpc,
              {'EXIT',
                  {noproc,
                      {gen_server,call,
                          [riak_core_cluster_manager,leader_node,infinity]}}}},
          #Fun<repl_util.27.120295566>],
         [{file,"src/rt.erl"},{line,510}]},
     {replication2,'-replication/3-fun-17-',2,
         [{file,"tests/replication2.erl"},{line,120}]},
     {replication2,replication,3,[{file,"tests/replication2.erl"},{line,120}]},
     {lists,foreach,2,[{file,"lists.erl"},{line,1323}]}]}
===============================================================================
```