Description of problem:
When AWS EBS CSI driver returns "RequestLimitExceeded" to ControllerUnpublishRequest (="detach"), csi-external-attacher marks the volume as detached:

I0805 16:05:08.948172       1 connection.go:180] GRPC call: /csi.v1.Controller/ControllerUnpublishVolume
I0805 16:05:08.948182       1 connection.go:181] GRPC request: {"node_id":"i-0eb51d1f90e270d91","volume_id":"vol-0766d9cc3c1966b86"}
I0805 16:05:15.323947       1 connection.go:183] GRPC response: {}
I0805 16:05:15.324406       1 connection.go:184] GRPC error: rpc error: code = Internal desc = Could not detach volume "vol-0766d9cc3c1966b86" from node "i-0eb51d1f90e270d91": could not detach volume "vol-0766d9cc3c1966b86" from node "i-0eb51d1f90e270d91": RequestLimitExceeded: Request limit exceeded.
        status code: 503, request id: 10ceab6c-4a6d-4da5-add2-d46d4fdb652a
I0805 16:05:15.324422       1 csi_handler.go:369] Detached "csi-51ad5c68abe99844c08cfce659c0f8375d8b0255391a6b28b543501577b3dee6" with error rpc error: code = Internal desc = Could not detach volume "vol-0766d9cc3c1966b86" from node "i-0eb51d1f90e270d91": could not detach volume "vol-0766d9cc3c1966b86" from node "i-0
eb51d1f90e270d91": RequestLimitExceeded: Request limit exceeded.
        status code: 503, request id: 10ceab6c-4a6d-4da5-add2-d46d4fdb652a
I0805 16:05:15.324459       1 util.go:70] Marking as detached "csi-51ad5c68abe99844c08cfce659c0f8375d8b0255391a6b28b543501577b3dee6"

The volume remains attached to a node and the external-attacher never re-tries to detach the volume.

Version-Release number of selected component (if applicable):
4.2.0-0.okd-2019-08-05-143844

How reproducible:
rarely

Steps to Reproduce:
1. run a pod with AWS EBS volume provided by CSI.
2. delete the pod and hope for RequestLimitExceeded response.

Actual results:
1. The volume is still attached to the node (as seen in AWS console)
2. VolumeAttachment Kubernetes object is deleted

Expected results:
1. The volume is detached from the node (after a while).
2. VolumeAttachment Kubernetes object is deleted.