Allow feeds without a valid pubDate to be added (e.g. LibriVox)

[tbradsha]

Expected behavior

Feeds without a pubDate should still be downloadable, perhaps with a warning that things will show in feed order rather than pub order (as one would expect for an audiobook like those in LibriVox). In my use case, the pubDate element was commented out.

At bare minimum, the error message should state why the feed cannot be added (e.g. missing pubDate!), but ideally it'd gracefully allow one to continue.

Actual behavior

One gets an error when trying to add the feed.

Steps to reproduce the behavior

Try adding the following feed:

Source: https://librivox.org/the-adventures-of-huckleberry-finn-version-5-dramatic-reading-by-mark-twain/ RSS: https://librivox.org/rss/9632 Modified RSS: https://pastebin.com/raw/m4SixrW2

Tested on web and iOS 15

This is a long-standing issue; see also: https://www.reddit.com/r/pocketcasts/comments/ekin6r/i_finally_found_ways_to_overcome_the_librivox/