download-lib.js: catch (ex) block calls console.error(err) (ReferenceError), masking the real download error

Bug Description

In download-lib.js, the catch block wrapping dl.start() references an undefined variable:

} catch (ex) {
    console.error(err);
    process.exit(1);
}

err is not defined in that scope (the parameter is ex), so whenever the download rejects, the script crashes with ReferenceError: err is not defined instead of printing the actual error. The other catch block in the same file uses console.error(ex) correctly, so this looks like a copy/paste slip. Present in @matrix-org/matrix-sdk-crypto-nodejs@0.6.6 and still on main (lines 82-83).

Impact

When the prebuilt binary download fails for any reason (network, CDN abort, TLS, proxy), pnpm install fails with only:

ReferenceError: err is not defined
    at download_lib (download-lib.js:83:23)

The real cause is completely hidden, which makes install failures very hard to diagnose. In our case the underlying error was a mid-transfer socket abort against the GitHub release-assets CDN; we only discovered it by patching this line locally to console.error(ex).

Steps to Reproduce

1. Make the binary download fail (e.g. block the download URL) and install any package running this postinstall, e.g. npm i @matrix-org/matrix-sdk-crypto-nodejs@0.6.6
2. Observe the failure output: only the ReferenceError above, no mention of the actual download error

Expected Behavior

The original download error is reported (e.g. console.error(ex)), not replaced by an unrelated ReferenceError.

Environment

• Package: @matrix-org/matrix-sdk-crypto-nodejs 0.6.6 (also unfixed on main)
• OS: Linux x64 (Ubuntu 24.04), Node v26.8.1

Start in download-lib.js around lines 82-83 and compare the catch block wrapping dl.start() with the other catch block in the same file. Reproduce a failed binary download during installation, then verify that the original download error is reported instead of a ReferenceError.