return no data error message to preview

[wnbts]

Current preview returns an internal exception when there is no data. For an example, see #28 This pr handles the case with a clear error message, as shown below.

[elasticsearch] java.lang.IllegalArgumentException: No data to preview anomaly detection.
[elasticsearch]         at com.amazon.opendistroforelasticsearch.ad.feature.FeatureManager.lambda$getPreviewFeatures$14(FeatureManager.java:277) [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:347) [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:196) [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]