Implements the optional parameter `returnPartialSuccess`.

If set to `true` and the API response contains an `unreachable` array,
the function will return a successful callback (`err: null`) and pass
the `unreachable` array as the 5th argument.

This allows users to process the available buckets without the entire
operation failing on soft errors.

New sample for partial success in ListBuckets
(storage_list_buckets_partial_success)