fix: surface embedding batch failures as warnings

Previously an error from the embedder would propagate up and abort the
entire index operation, leaving no record of what failed. Wrap each
batch in try/catch: record the failure in IndexResult.errors and print
a warning to stderr so the user sees a partial index rather than a
crash.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>