Fixed with https://github.com/openshift/origin-server/pull/4241

The message now contains which ops failed.

Note : The rhc client tool does not give the 'debug' messages as contained in the REST output, perhaps we need the client tool to be enhanced?

e.g. if a node times out processing a parallel request, the REST response will contain a debug message like : (Gear Id: 52136834) Execution expired.
But the 'rhc' tool does not show it.

Verified on devenv_4067

Modify pending_app_op_group.rb as following:
failed_ops << tag["op_id"] # if status != 0

Then restart services and clear broker cache. 
Create one application via RESTAPI
curl -s -k -H 'Content-Type: Application/json' --user jhou@redhat.com:x https://ec2-23-22-167-139.compute-1.amazonaws.com/broker/rest/domains/jhou/applications/ -X POST -d '{"name":"d1", "cartridge":"diy-0.1"}'

Result:
The ops class will be displayed to tell user
{
    "api_version": 1.6,
    "data": null,
    "messages": [
        {
            "exit_code": 0,
            "field": null,
            "index": null,
            "severity": "result",
            "text": "Disclaimer: This is an experimental cartridge that provides a way to try unsupported languages, frameworks, and middleware on OpenShift.\n"
        },
        {
            "exit_code": 0,
            "field": null,
            "index": null,
            "severity": "error",
            "text": "Unable to complete the requested operation due to: Failed to correctly execute all parallel operations - [\"UpdateAppConfigOp\", \"AddBrokerAuthKeyOp\"].\nReference ID: 2a187f2946964ce241e5468576ad695d"
        }
    ],
    "status": "internal_server_error",
    "supported_api_versions": [
        1.0,
        1.1,
        1.2,
        1.3,
        1.4,
        1.5,
        1.6
    ],
    "type": null,
    "version": "1.6"
}