Add logging when monitoring cannot connect (elastic#6365)

Currently when the monitoring can not connect to Elasticsearch no errors
is show in the log making this issue really hard to debug, this change
the behavior to send any error to the log.

Fixes: elastic#6327

(cherry picked from commit b4c92f8)

libbeat/monitoring/report/elasticsearch/elasticsearch.go

@@ -161,6 +161,8 @@ func (r *reporter) initLoop() {
 		if err == nil {
 			closing(client)
 			break
+		} else {
+			logp.Err("Monitoring could not connect to elasticsearch, failed with %v", err)
 		}
 
 		select {