Repeating post is marked as such even though a Repeat post is not created for it (due to be scheduled in the past)

A Repeat post is created only if date of a Repeating post + it's schedule is in the future.

When you setup a post to be a Repeating post with a schedule such that a Repeat post date happens to be in the past (read example below), the Repeat post is not created. However, the Repeating post is marked as `Repeating`. This is confusing!

**Solution**: an error message should be displayed in such case while trying to save a post with a Repeating schedule. That post won't be saved as `Repeating` post.

Example: main post (that we're wanting to set as a Repeating post) is published on the 10th October and we're setting it to be a `Daily` Repeating post. So a daily repeat post would have been scheduled for the 11th October. However, if present time (when we're saving changes) is anything past the 11th October, say the 12th October - then Repeat post date is in the past and therefore it is not published.

The issue names no files or tests. Start by tracing the save path for Repeating posts and the logic that calculates the first Repeat post date, then reproduce the daily-schedule example with the repeat date already in the past. Done means saving is rejected with an error and the post is not marked as Repeating.

索引モデルが issue の本文から書いたものです。