Add enumerated failure reasons to gRPC response structures

[scottbrewer]

Hi team -

This is an enhancement request to the gRPC proto that would return an enumerated detailed failure reason on gRPC requests that cannot be completed successfully. Today a boolean status field is all that exists in most response types, but a failed/false status does not give the client/user any indication of what went wrong. It would be nice to be able to determine the difference between a create request that failed because an input parameter was malformed versus because the resource already exists (can be treated as success in some cases).

I recommend the following as a minimum:

enum StatusReason {
    STATUS_REASON_NO_ERROR = 0
    STATUS_REASON_BAD_REQUEST = 1
    STATUS_REASON_RESOURCE_EXISTS = 2
    STATUS_REASON_RESOURCE_NOT_FOUND = 3
    STATUS_REASON_RESOURCE_BUSY = 4
    STATUS_REASON_INTERNAL_ERROR = 5
}

Once the enum list is created, all response types should be updated to include a status and status reason (most have status already) and the server can be updated to set the fields appropriately in error paths.

Thanks

[caroav]

@oritwas please add your input here.

[oritwas]

We should use existing standard status codes and not invent our own. We can use the grpc error codes,
https://grpc.github.io/grpc/core/md_doc_statuscodes.html or the standard Linux error codes

[gbregman]

This was implemented in PRs #343 and #352. The status returned by gRPC is now an errno code. Notice, that in most cases we don't know the exact problem as it is internal to SPDK. We just get a boolean status from SPDK whether the operation was successful or not.