[postprocessor] update 'finalize' events

Add 'finalize-error' and 'finalize-success' events that trigger
depending on whether error(s) did or did not happen.

'finalize' itself now always triggers regardless of error status.
(was supposed to have the same behavior as the new 'finalize-success')

docs/configuration.rst

@@ -4512,6 +4512,10 @@ Description
         and before the first file download
     ``finalize``
         On extractor shutdown, e.g. after all files were downloaded
+    ``finalize-success``
+        On extractor shutdown when no error occurred
+    ``finalize-error``
+        On extractor shutdown when at least one error occurred
     ``prepare``
         Before a file download
     ``file``