return empty preview results on failure

[wnbts]

When previewing anomaly results encounters an issue, the response is empty instead of an exception to the kibana frontend and the cause is logged with info.

[elasticsearch] [2020-01-14T18:20:00,979][INFO ][c.a.o.a.AnomalyDetectorRunner] [node-0] Fail to preview anomaly detector W0n_pm8B_IjOQi-ITIdN
[elasticsearch] java.lang.IllegalArgumentException: No data to preview anomaly detection.
[elasticsearch]         at com.amazon.opendistroforelasticsearch.ad.feature.FeatureManager.lambda$getPreviewFeatures$14(FeatureManager.java:296) [opendistro-anomaly-detection-1.2.1.0-alpha.jar:1.2.1.0-alpha]
[elasticsearch]         at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:62) [elasticsearch-7.2.1.jar:7.2.1]
[elasticsearch]         at com.amazon.opendistroforelasticsearch.ad.feature.FeatureManager.lambda$getSamplesForRanges$19(FeatureManager.java:370) [opendistro-anomaly-detection-1.2.1.0-alpha.jar:1.2.1.0-alpha]
[elasticsearch]         at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:62) [elasticsearch-7.2.1.jar:7.2.1]
[elasticsearch]         at com.amazon.opendistroforelasticsearch.ad.feature.SearchFeatureDao.lambda$getFeatureSamplesForPeriods$24(SearchFeatureDao.java:216) [opendistro-anomaly-detection-1.2.1.0-alpha.jar:1.2.1.0-alpha]
[elasticsearch]         at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:62) [elasticsearch-7.2.1.jar:7.2.1]
[elasticsearch]         at org.elasticsearch.action.support.TransportAction$1.onResponse(TransportAction.java:68) [elasticsearch-7.2.1.jar:7.2.1]
[elasticsearch]         at org.elasticsearch.action.support.TransportAction$1.onResponse(TransportAction.java:64) [elasticsearch-7.2.1.jar:7.2.1]
[elasticsearch]         at org.elasticsearch.action.search.TransportMultiSearchAction$1.finish(TransportMultiSearchAction.java:177) [elasticsearch-7.2.1.jar:7.2.1]
[elasticsearch]         at org.elasticsearch.action.search.TransportMultiSearchAction$1.handleResponse(TransportMultiSearchAction.java:163) [elasticsearch-7.2.1.jar:7.2.1]
[elasticsearch]         at org.elasticsearch.action.search.TransportMultiSearchAction$1.onResponse(TransportMultiSearchAction.java:151) [elasticsearch-7.2.1.jar:7.2.1]
[elasticsearch]         at org.elasticsearch.action.search.TransportMultiSearchAction$1.onResponse(TransportMultiSearchAction.java:148) [elasticsearch-7.2.1.jar:7.2.1]