Remove hack to use warnings with glib development versions

Do not assert_not_reached() on errors, but handle timeouts with retry
and other errors like other parsing errors

If we still have filtered contexts in shutdown, drop that reference as
well

- Also free list if the manager does not live anymore
- Free the list nodes properly

- Unref the actual context, not the list
- Free the list element

Properly unref cancellable when being done fetching description
document.

Trying to propagate an error over some unintialized error causes a
warning and, more importantly, an invalid free

Fixes #80

Fixed #81