[postprocessor/ffmpeg] Show ffmpeg output on error (refs #22680) (#29336)

Author: kikuyan

youtube_dl/postprocessor/ffmpeg.py

@@ -231,7 +231,10 @@ def run_ffmpeg_multiple_files(self, input_paths, out_path, opts):
         stdout, stderr = p.communicate()
         if p.returncode != 0:
             stderr = stderr.decode('utf-8', 'replace')
-            msg = stderr.strip().split('\n')[-1]
+            msgs = stderr.strip().split('\n')
+            msg = msgs[-1]
+            if self._downloader.params.get('verbose', False):
+                self._downloader.to_screen('[debug] ' + '\n'.join(msgs[:-1]))
             raise FFmpegPostProcessorError(msg)
         self.try_utime(out_path, oldest_mtime, oldest_mtime)