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(

src/main/log.ts

@@ -12,3 +12,44 @@ export function cleanError(stderr: string): string {
   const errLine = [...lines].reverse().find((l) => /^error/i.test(l))
   return (errLine ?? lines[lines.length - 1] ?? '').replace(/^error:\s*/i, '').trim()
 }
+
+// Lines that carry no diagnostic value as post-processing context: per-tick
+// download progress and the "Deleting original file" cleanup bookkeeping yt-dlp
+// prints between merge steps.
+const CONTEXT_NOISE = /^\[download\]|^Deleting original file/i
+// A single context line, capped so a long output path can't bloat the message.
+const CONTEXT_LINE_MAX = 200
+
+/**
+ * Richer error for the DOWNLOAD failure path (probe/indexer/updater keep the
+ * concise cleanError line).
+ *
+ * yt-dlp collapses an ffmpeg post-processing failure to a bare
+ * "ERROR: Postprocessing: Conversion failed!" that names neither the step that
+ * died nor the reason — and without `--verbose` the underlying ffmpeg detail is
+ * never emitted at all. What yt-dlp DOES print is a tagged progress line per
+ * post-processor ([Merger], [ModifyChapters], [ExtractAudio], …; its phase tags
+ * like [download]/[info] are lowercase). So for that generic wrapper we append
+ * the last couple of non-noise trailing lines, which name the failing step (e.g.
+ * the SponsorBlock re-encode) and surface any ffmpeg diagnostic when one is
+ * present. Self-explanatory errors (a private video, a 403) are returned
+ * unchanged.
+ */
+export function describeDownloadError(stderr: string): string {
+  const headline = cleanError(stderr)
+  if (!headline || !/conversion failed|postprocessing/i.test(headline)) return headline
+  const lines = stderr
+    .split('\n')
+    .map((l) => l.trim())
+    .filter(Boolean)
+  const context: string[] = []
+  for (const line of [...lines].reverse()) {
+    if (context.length >= 2) break
+    if (CONTEXT_NOISE.test(line) || /^error/i.test(line)) continue
+    // Skip anything already conveyed by the headline (avoids echoing it back).
+    if (headline.includes(line) || line.includes(headline)) continue
+    const capped = line.length > CONTEXT_LINE_MAX ? `${line.slice(0, CONTEXT_LINE_MAX - 1)}…` : line
+    if (!context.includes(capped)) context.unshift(capped)
+  }
+  return context.length > 0 ? `${headline} — ${context.join(' · ')}` : headline
+}