ClusterRuntime un-getable

### What happened?

It looks like with recent changes to the Kubernetes backend, ClusterRuntime are not retrievable.

When attempting to get a ClusterRuntime, it can never go past the exception caught on L199:
https://github.com/kubeflow/sdk/blob/db76de0ca8c32540b8a177a713e73f41c1bf81af/kubeflow/trainer/backends/kubernetes/backend.py#L174-L223

The change seems to come from https://github.com/kubeflow/sdk/pull/307. Based on skimming that PR, it looks like it probably worked originally with the `pass` but that was pulling out due to a suggestion from copilot. It doesn't need to raise the error there as that requires something above it to catch it. It should probably just print/log it instead of catching and raising again.

Disclaimer: I am not familiar with this codebase nor have I had time to fully dig into it, so maybe I missed up something in my configuration. Either way, it warrants a deeper look.

### What did you expect to happen?

`get_runtime` should first try namespaced runtimes and then fallback to cluster runtimes.

### Environment

Kubernetes version:
```bash
$ kubectl version

```
Kubeflow Trainer version:
```bash
$ kubectl get pods -n kubeflow -l app.kubernetes.io/name=trainer -o jsonpath="{.items[*].spec.containers[*].image}"

```
Kubeflow Python SDK version:
```bash
$ pip show kubeflow

```

### Impacted by this bug?

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Start in kubeflow/trainer/backends/kubernetes/backend.py around lines 174-223 and trace get_runtime's namespaced lookup, exception handling, and cluster-runtime fallback. Compare the behavior with the change from pull request #307. Done means get_runtime first tries namespaced runtimes and then successfully falls back to cluster runtimes without the lookup exception preventing retrieval.

Written by the indexing model from the issue text.