void queueFull() {
        requestContext.processingFailed(RequestContext.ErrorCode.ERROR_BUSY,
                                        "Queue temporarily full. Returning message " + requestContext +
                                        ". Will be automatically resent.");
    }
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