fix(download): name the failing step in postprocessing errors

yt-dlp collapses an ffmpeg postprocessing failure to a bare
"Postprocessing: Conversion failed!" with no indication of which step
died, and without --verbose the underlying ffmpeg reason is never
emitted at all. describeDownloadError() appends the last non-noise
trailing lines (e.g. the [ModifyChapters] SponsorBlock re-encode) so
the error log and completion notification point at the actual cause
instead of a dead end.

src/main/download.ts

@@ -33,7 +33,7 @@ import {
   FILEPATH_MARKER,
   DEST_MARKER
 } from './core/buildArgs'
-import { cleanError } from './log'
+import { describeDownloadError } from './log'
 import { addErrorLog } from './errorlog'
 import {
   STALL_TIMEOUT_MS,
@@ -530,7 +530,7 @@ function wireChildProcess(params: {
         opts.incognito
       )
     } else {
-      const msg = cleanError(stderrTail) || `yt-dlp exited with code ${code}`
+      const msg = describeDownloadError(stderrTail) || `yt-dlp exited with code ${code}`
       send(wc, { type: 'error', id: opts.id, error: msg })
       logFailure(opts, getTitle(), msg)
       notify(