Watcher does not handle an error.

## Environment details:

* FPCScheduler

## Steps to reproduce the issue:

The watcher service uses `watchAllKeys` method to initialize the watcher.

```scala
private val watcher = etcdClient.watchAllKeys { res: WatchUpdate =>
res.getEvents.asScala.foreach { event =>
event.getType match {
case EventType.DELETE =>
val key = ByteStringToString(event.getPrevKv.getKey)
val value = ByteStringToString(event.getPrevKv.getValue)
val watchEvent = WatchEndpointRemoved(key, key, value, false)
deleteWatchers
.foreach { watcher =>
if (watcher._1.watchKey == key) {
watcher._2 ! watchEvent
}
}
prefixDeleteWatchers
.foreach { watcher =>
if (key.startsWith(watcher._1.watchKey)) {
watcher._2 ! WatchEndpointRemoved(watcher._1.watchKey, key, value, true)
}
}
case EventType.PUT =>
val key = ByteStringToString(event.getKv.getKey)
val value = ByteStringToString(event.getKv.getValue)
val watchEvent = WatchEndpointInserted(key, key, value, false)
putWatchers
.foreach { watcher =>
if (watcher._1.watchKey == key) {
watcher._2 ! watchEvent
}
}
prefixPutWatchers
.foreach { watcher =>
if (key.startsWith(watcher._1.watchKey)) {
watcher._2 ! WatchEndpointInserted(watcher._1.watchKey, key, value, true)
}
}
case msg =>
logging.debug(this, s"watch event received: $msg.")
}
}

}
```

The `watchAllKeys` method has an error handler in the argument, but no error handler is provided.

```scala
def watchAllKeys(next: WatchUpdate => Unit = (_: WatchUpdate) => {},
error: Throwable => Unit = (_: Throwable) => {},
completed: () => Unit = () => {}): Watch = {
client.getKvClient
.watch(KvClient.ALL_KEYS)
.prevKv()
.executor(threadpool)
.start(new StreamObserver[WatchUpdate]() {
override def onNext(value: WatchUpdate): Unit = {
next(value)
}

override def onError(t: Throwable): Unit = {
error(t)
}

override def onCompleted(): Unit = {
completed()
}
})
}
```

So the default error handler(`(_: Throwable) => {}`) is being used and it does nothing.

## Provide the expected results and outputs:
The error handler should print an error logs and reconnect to ETCD in case of any connection error to restore watcher.

This might be related to https://github.com/apache/openwhisk/issues/5286