The current error comparison `imagePullResult.err ==
ErrRegistryUnavailable` will never work with any remote runtime, because
we produce gRPC errors which wrap a code and a description, like:
```
rpc error: code = Unknown desc = This is the error description
```
To be able to check custom error types from `pkg/kubelet/images/types.go`,
we now strip the code if the status is unknown on image pull.
Beside that, we use a string comparison to check against
`ErrRegistryUnavailable.Error()`, because validating them via the
`errors` package is not yet supported by grpc-go:
https://github.com/grpc/grpc-go/issues/3616
Signed-off-by: Sascha Grunert <sgrunert@redhat.com>
4cdfe600e0