Post update doesn't fail cleanly when ACF fields are invalid

[TobyWilkes]

Describe the bug
When saving a post using gutenberg which has custom fields, or some other field requiring validation, if the validation fails the request to "wp-admin/post.php" will return 500. However the UI will remain locked in "saving", making it impossible to update the contents or retry saving any further.

Further, a "Post updated!" notification appears, even though the post update has failed.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'Edit Post' on a site with required custom fields.
2. Leave some of the fields empty
3. Press 'Update'
4. See incorrect "Post updated!" notice and be locked in "Updating..." state forever.

Expected behavior
When the request to "post.php" fails, some error should be displayed at best. At worst the UI should fall out of the "Updating..." state and allow the user to edit the contents again and retry updating.

The UI reads "Post Updated!" even though it is not. This shouldn't be displayed.

Screenshots


(erroneous 'Post Updated!' notification and locked 'Saving...' state)

Desktop:

• OS: MacOS High Sierra
• Browser: Firefox
• Version: 62.0.2

Additional context

• Gutenberg 3.8

[designsimply]

Hi! I think this is a duplicate of #9699. Please have a look and if it's not the same please reply here.