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

### 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/

No source files, tests, or entry points are named. Reproduce the add-feed failure with the modified LibriVox RSS and trace the validation path; done means feeds missing pubDate can be added with the stated ordering warning, or the error clearly explains the missing field.

Written by the indexing model from the issue text.