make election stop not be a failure (#19705)

During our master elections, nodes "vote" for a master being issuing a join request to it. Since this is done in an async fashion, joins may arrive before the master itself has realized it had won the election. Therefore we start accumulating node joins on every node at election start (we don't know the result yet). When the election finish nodes that did not become the master (i.e., joined another node which won the election) need to potentially process and fail any incoming join request they may have received during the election. This is currently achieved by always issuing a cluster state update task that is doomed to fail, even if no pending joins are actually there. That aspect results in confusing (debug) log messages, making it seems like something is wrong. For example (note that `NotMasterException`)

```
[2016-07-30 22:25:53,040][DEBUG][cluster.service          ] [node_t1] processing [zen-disco-process-pending-joins [{node_t0}{4SqBTyYNQ82J9c75Cs7jtg}{kutaNSYbTZCSybvqczgWCA}{127.0.0.1}{127.0.0.1:9400} elected]]: execute
[2016-07-30 22:25:53,041][DEBUG][transport                ] [node_t1] connected to node [{node_t0}{4SqBTyYNQ82J9c75Cs7jtg}{kutaNSYbTZCSybvqczgWCA}{127.0.0.1}{127.0.0.1:9400}]
[2016-07-30 22:25:53,045][DEBUG][cluster.service          ] [node_t1] cluster state update task [zen-disco-process-pending-joins [{node_t0}{4SqBTyYNQ82J9c75Cs7jtg}{kutaNSYbTZCSybvqczgWCA}{127.0.0.1}{127.0.0.1:9400} elected]] failed
NotMasterException[Node [{node_t1}{eAQts270TiGFpoCDE-0PQQ}{or5bsv2ET220su78DLJk5g}{127.0.0.1}{127.0.0.1:9401}] not master for join request]
[2016-07-30 22:25:53,048][DEBUG][cluster.service          ] [node_t1] processing [zen-disco-process-pending-joins [{node_t0}{4SqBTyYNQ82J9c75Cs7jtg}{kutaNSYbTZCSybvqczgWCA}{127.0.0.1}{127.0.0.1:9400} elected]]: took [7ms] no change in cluster_state
```

This commit cleans up  the logic a bit to only use failure where there are actual joins that are failed. The result is cleaner logs as well:

```
[2016-07-30 22:23:12,880][DEBUG][cluster.service          ] [node_t1] processing [zen-disco-election-stop [{node_t0}{jMR5HCpOQnOM4pGeFkUjng}{B5WIZQAdQk2cWbjGZ21mvQ}{127.0.0.1}{127.0.0.1:9400} elected]]: execute
[2016-07-30 22:23:12,881][DEBUG][cluster.service          ] [node_t1] processing [zen-disco-election-stop [{node_t0}{jMR5HCpOQnOM4pGeFkUjng}{B5WIZQAdQk2cWbjGZ21mvQ}{127.0.0.1}{127.0.0.1:9400} elected]]: took [0s] no change in cluster_state
[2016-07-30 22:23:12,881][DEBUG][transport                ] [node_t1] connected to node [{node_t0}{jMR5HCpOQnOM4pGeFkUjng}{B5WIZQAdQk2cWbjGZ21mvQ}{127.0.0.1}{127.0.0.1:9400}]
```

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