Display embeds error message only when the form is submitted

[imath]

Testing Gutenberg 4.0.0-RC1 I noticed a confusing behavior about the Embeds blocks. As soon as you enter some content into the URL input there is an error message that is added under the URL input no matter if the URL is actually embeddable. I would suggest to wait untill the form has been submitted to eventually display this error. Otherwise some users like me might be confused to see this error although the URL is embeddable.

PS: this PR should fix #10355

Description

I've added a new property to the Embed block's state to track whether the form has been submitted or not.

How has this been tested?

I've tested using this URL https://vimeo.com/287192106 which is an embeddable one within the Gutenberg Editor on WordPress trunk (as Gutenberg is not working on branch 5.0 of WordPress ??).
I've also ran the tests suite.

Screenshots

The error message

The error message shows even if the URL is embeddable as the above GIF proves it.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[youknowriad]

@notnownikki Does this ring any bell for you? Apparently, it's a 4.0 regression?

[notnownikki]

Yeah, this was fixed by #10467 which finally moved the logic into the withSelect part instead of having it peppered over a few places in the edit component.

[gziolo]

Let's wait for 4.1 to get this fixed with a much aggressive fix performed in #10467 by @notnownikki. Otherwise, we risk some bigger conflicts to have to resolve in both 4.0-RC and master.

Thanks @imath for spending your time on this PR. I will close it for now. We can always reopen if we change the way we publish 4.0 - by the way, it should happen today.