[Data] Add AWS Error UNKNOWN to list of retried write errors (#46646)

When writing Parquet files to S3, you might see a transient error like this:

```
AWS Error UNKNOWN (HTTP status 503) during CreateMultipartUpload operation: No response body
```

Currently, this error isn't retried, and your program crashes.

Signed-off-by: Balaji Veeramani <[email protected]>

python/ray/data/context.py

@@ -90,6 +90,7 @@
     "AWS Error INTERNAL_FAILURE",
     "AWS Error NETWORK_CONNECTION",
     "AWS Error SLOW_DOWN",
+    "AWS Error UNKNOWN (HTTP status 503)",
 )
 
 DEFAULT_WARN_ON_DRIVER_MEMORY_USAGE_BYTES = 2 * 1024 * 1024 * 1024