@Override
    public String toString() {
        return "ProcessingTask{" +
               "processings=" + processings +
               ", processingsDone=" + processingsDone +
               ", requestContext=" + requestContext +
               '}';
    }
Processes a single Processing, and fails the message if this processing fails.

@param executor the DocprocService to use for processing
docproc/src/main/java/com/yahoo/docproc/jdisc/DocumentProcessingTask.java