OPENDJ-1762 (CR-5923) verify-index on non existing index fails with NullPointerException when the server is stopped

Verifying a non existing index triggers some untested unhappy paths.
Faulty exception handling was creating the issue.


PersistItStorage.java:
In read(), only throw the cause if there is a cause.

VerifyJob.java:
Do not wrap StorageRuntimeException inside StorageRuntimeException.

BackendImpl.java:
Chain the original exception.