Better error message and recovery when indexing doesn't work

[wolfv]

When there is a broken .conda file in the output folder, we currently print an error that does not indicate which package is the culprit.

We should probably also just ignore the package.

This is what it looks like right now:

 ╭─ Running build for recipe: whisper-cpp-base-en-0.1.0-h4616a5c_0
thread 'main' panicked at /Users/runner/miniforge3/conda-bld/rattler-build_1713389831420/_build_env/.cargo/registry/src/index.crates.io-6f17d22bba15001f/rattler_index-0.19.6/src/lib.rs:73:55:
Could not open conda file: ZipError(InvalidArchive("Could not find central directory end"))
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
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