[FLINK-37831] V3.1 do not retry failed index operations

[thbay]

When using flink-connector-elasticsearch8 ver. 3.1.0-1.20 with flink ver. 1.20.1,
the retry of failed index operations has two problems:

1. the isRetryable method is testing error.getCause() which is null in case of ConnectException and other exceptions
2. ELASTICSEARCH_FATAL_EXCEPTION_CLASSIFIER.isFatal() method returns true if exception is not thrown, resulting in the isRetryable() method returns the wrong result.

The committed fix works for me.

[reswqa]

cc @mtfelisb