disk/ad_slot: return better GRPC error message

[huww98]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

expected new message shown in Kubernetes events:
failed to reserve node i-xxxx for attach: still waiting for other disk(s) to finish attach/detach

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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[huww98]

I've verified that I can see the expected Kubernetes event with this patch. However, this path is hard to trigger, because requests are not enqueued at the same time. when the currently attaching request is cancelled, the next one kicks in and issue AttachDisk call immediately, and likely receive Conflict response. So, these conflicting AttachDisk calls can be avoided.

Maybe something like #1190 can be helpful, but still not ideal. Because we don't want to delay the GRPC response to avoid potential timeout.

That said, this issue is not introduced by this PR. So I think this one is ready to go.