Separate the options --ignore-errors and --no-abort-on-error

In youtube-dl, `-i` ignores both download and post-processing error, and
treats the download as successful even if the post-processor fails.

yt-dlp used to skip the entire video on either error and there was no
option to ignore the post-processing errors like youtube-dl does.

By splitting the option into two, now either just the download errors
(--no-abort-on-error, default on CLI) or all errors (--ignore-errors)
can be ignored as per the users' needs

Closes #893

yt_dlp/postprocessor/ffmpeg.py

@@ -288,8 +288,7 @@ class FFmpegPostProcessor(PostProcessor):
         stdout, stderr = process_communicate_or_kill(p)
         if p.returncode not in variadic(expected_retcodes):
             stderr = stderr.decode('utf-8', 'replace').strip()
-            if self.get_param('verbose', False):
-                self.report_error(stderr)
+            self.write_debug(stderr)
             raise FFmpegPostProcessorError(stderr.split('\n')[-1])
         for out_path, _ in output_path_opts:
             if out_path: