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/common.py

@@ -52,6 +52,7 @@ class PostProcessor(object):
             return self._downloader.report_warning(text, *args, **kwargs)
 
     def report_error(self, text, *args, **kwargs):
+        # Exists only for compatibility. Do not use
         if self._downloader:
             return self._downloader.report_error(text, *args, **kwargs)