Describe the bug
When a user configures an S3 bucket that doesn't support pruning directories, they only see unexpected_error_occurred in the UI when trying to publish to the bucket. We should display a clearer error message so the user can resolve this issue themselves without needing access to the server logs.

To Reproduce
Steps to reproduce the behavior:

1. Create an S3 bucket using a non-official 3S protocol implementation that does not support pruning.
2. Enable pruning for publishing files into that bucket.
3. Try to publish manually from the UI.
4. Observe the unexpected_error_occurred error.

Expected behavior
The error message should indicate something like "Prune of S3 bucket failed."