Block Validation: Further improve deprecations warnings for nested blocks

[gziolo]

Follow-up for #16862 where landed changes that prevent showing errors and warnings when blocks get successfully updated from the deprecated version. Ther is still this edge case which needs to be resolved for nested blocks as described by @mcsf in #16862 (review):

The flow in the log can be a bit awkward depending on what is valid and invalid. Steps to repro:

1. Change the demo content by adding a bogus data attribute to the first DIV, like so:

diff --git a/post-content.php b/post-content.php
index 032c4ad10..7f3621ada 100644
--- a/post-content.php
+++ b/post-content.php
@@ -7,7 +7,7 @@
 
 ?>
 <!-- wp:cover {"url":"https://cldup.com/Fz-ASbo2s3.jpg","className":"alignwide"} -->
-<div class="wp-block-cover has-background-dim alignwide" style="background-image:url(https://cldup.com/Fz-ASbo2s3.jpg)"><div class="wp-block-cover__inner-container"><!-- wp:paragraph {"align":"center","placeholder":"Write title…","fontSize":"large"} -->
+<div class="wp-block-cover has-background-dim alignwide" data-foo="bar" style="background-image:url(https://cldup.com/Fz-ASbo2s3.jpg)"><div class="wp-block-cover__inner-container"><!-- wp:paragraph {"align":"center","placeholder":"Write title…","fontSize":"large"} -->
 <p style="text-align:center" class="has-large-font-size"><?php _e( 'Of Mountains &amp; Printing Presses', 'gutenberg' ); ?></p>
 <!-- /wp:paragraph --></div></div>
 <!-- /wp:cover -->

2. This will break the opening Cover block of the demo.

3. Open the demo post. Notice how the first line logged reads Block successfully updated for core/paragraph, which refers to the P tag inside the broken Cover block. The following lines are errors logged for the invalid Cover block as a whole.

4. The first line, about core/paragraph, should probably not be there, since in the end we didn't get a new Paragraph block in the block list, we only get an invalid Cover block.

[paaljoachim]

Hi Greg @gziolo
Is this issue still valid?

[gziolo]

I don't see any referenced PRs, so it still has to be implemented. It's going to become a more evident issue when we see more FSE themes since they use nested blocks extensively.

[paaljoachim]

I just added the Full Site Editing label to it as this as you say impacts FSE themes.