Fixed: DLCheckpointStore rethrows exceptions as FNFE but swallows the rootcause

[dlg99]

DLCheckpointStore rethrows exceptions as FNFE but swallows the rootcause

Descriptions of the changes in this PR:

added .initCause for the new FNFE exception to preserver it

Motivation

got the
ERROR org.apache.bookkeeper.statelib.impl.rocksdb.checkpoint.RocksCheckpointer - Failed to restore rocksdb ...
java.io.FileNotFoundException: ..../metadata
without any additional info for the rootcause (other than stack pointing to the DLCheckpointStore)

DLCheckpointStore appears to re-throw exceptions as FNFE without preserving the rootcause

Changes

added .initCause

Master Issue: #2552

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[eolivelli]

Lgtm