fix: handle relative index URLs without panicking

[giri256]

Fixes #210

download_vidx() parsed each index URL with unwrap(). A relative URL therefore panicked the update worker before the surrounding retry-and-skip logic could handle the malformed entry.

This propagates the URL parse error through the function's existing Result instead. The caller can then retry and eventually ignore the bad URL without aborting the whole index update.

The regression test reproduces the previous RelativeUrlWithoutBase panic and verifies that the same input now returns an error.

Verification:

• cargo test -p cmsis-pack update::download::tests::relative_vidx_url_returns_error -- --exact
• cargo fmt --all -- --check