Allow backends to return partial success for multistatus responses

Allow backends to return a list of results alongside a list of HTTP errors.

Need a new error type holding an HTTP error + the resource path.

Start by tracing how backends currently report results and HTTP errors. Define done as supporting both result and error lists for multistatus responses, with each new error carrying an HTTP error and resource path, then verify the behavior with the repository’s existing tests.

Written by the indexing model from the issue text.