Note how the `onerror` functionality is not being used in the GENERIC `DownloadManager`, since we have no way of knowing if downloading succeeded.
Hence this functionality is only *possibly* useful in MOZCENTRAL builds, however as outlined in the existing comments it's unlikely to be helpful in practice. Generally speaking, if downloading failed once in [`PdfStreamConverter.jsm`](https://searchfox.org/mozilla-central/rev/809ac3660845fef6faf18ec210232fdadc0f1ad9/toolkit/components/pdfjs/content/PdfStreamConverter.jsm#294-406) it seems very likely that it would fail again; all-in-all I'm thus suggesting that we just remove the `onerror` functionality altogether here.