[ci][RLlib] [test utils] Improve error message on some test-failures because of IndexError in test_utils.py (#54343)

## Why are these changes needed?

In the CI I saw a fleaky test failing, but its error is not reported
correctly:

```python
[2025-07-04T00:01:57Z] Traceback (most recent call last):
[2025-07-04T00:01:57Z]   File "/root/.cache/bazel/_bazel_root/1df605deb6d24fc8068f6e25793ec703/execroot/com_github_ray_project_ray/bazel-out/k8-opt/bin/rllib/examples/connectors/multi_agent_observation_preprocessor.runfiles/com_github_ray_project_ray/rllib/examples/connectors/multi_agent_observation_preprocessor.py", line 137, in <module>
[2025-07-04T00:01:57Z]     run_rllib_example_script_experiment(base_config, args)
[2025-07-04T00:01:57Z]   File "/rayci/python/ray/rllib/utils/test_utils.py", line 1354, in run_rllib_example_script_experiment
[2025-07-04T00:01:57Z]     f"{[e.args[0].args[2] for e in results.errors]}"
[2025-07-04T00:01:57Z]   File "/rayci/python/ray/rllib/utils/test_utils.py", line 1354, in <listcomp>
[2025-07-04T00:01:57Z]     f"{[e.args[0].args[2] for e in results.errors]}"  # <---
[2025-07-04T00:01:57Z] IndexError: tuple index out of range  # <---
```

In the logs the actual error does not appear because of the `IndexError`
for ` f"{[e.args[0].args[2] for e in results.errors]}"`. I am not sure
why `e.args[0].args[2]` is used, a nested Exception perhaps? This PR
adds a guard to fallback to a simpler `repr(e)` in case the `IndexError`
would accur again. This allows to investigate why this `IndexError`
occures and raises the `RuntimeError` like expected.

## Related issue number

NA

## Checks

- [x] I've signed off every commit(by using the -s flag, i.e., `git
commit -s`) in this PR.
- [x] I've run `scripts/format.sh` to lint the changes in this PR.
- [x] I've made sure the tests are passing. Note that there might be a
few flaky tests, see the recent failures at https://flakey-tests.ray.io/
- Testing Strategy
   - [x] This PR is not tested, but improves other tests :)

---------

Signed-off-by: Daraan <github.blurry@9ox.net>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Kamil Kaczmarek <kaczmarek.poczta@gmail.com>
Signed-off-by: elliot-barn <elliot.barnwell@anyscale.com>

Author: 3 people

rllib/utils/test_utils.py

@@ -1362,11 +1362,17 @@ def run_rllib_example_script_experiment(
 
     # Error out, if Tuner.fit() failed to run. Otherwise, erroneous examples might pass
     # the CI tests w/o us knowing that they are broken (b/c some examples do not have
-    # a --as-test flag and/or any passing criteris).
+    # a --as-test flag and/or any passing criteria).
     if results.errors:
+        # Might cause an IndexError if the tuple is not long enough; in that case, use repr(e).
+        errors = [
+            e.args[0].args[2]
+            if e.args and hasattr(e.args[0], "args") and len(e.args[0].args) > 2
+            else repr(e)
+            for e in results.errors
+        ]
         raise RuntimeError(
-            "Running the example script resulted in one or more errors! "
-            f"{[e.args[0].args[2] for e in results.errors]}"
+            f"Running the example script resulted in one or more errors! {errors}"
         )
 
     # If run as a test, check whether we reached the specified success criteria.