Serialize onCompleteFailure for #9059 (#9062)

Serialize onCompleteFailure for #9059

* Fixed case where process() throws an exception.
   Before, exiting the processing loop would always skip to invoke onCompleteFailure(), causing the callback to not be completed.
   Now we fall through and possibly invoke onCompleteFailure() if it was not already invoked.

* Updated javadocs.

* Code cleanups.

Co-authored-by: Simone Bordet <simone.bordet@gmail.com>