Add error message under input field.

[PARTHVATALIYA]

Trac ticket: https://core.trac.wordpress.org/ticket/60969

Add error message to URL input field under the add custom link section.
Add CSS to the error message.

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Props parthvataliya, joedolson.

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[github-actions]

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[joedolson]

There are also some misc. code style issues in the JS, mostly surrounding use of spaces - in general the content inside parentheses should have spaces, e.g. $( '#submit-customlinkdiv' ).

But this is generally good; just needs some minor polish.

[PARTHVATALIYA]

@joedolson I made the changes you suggested can you please review it.

Thank You!

Outdated; completed but not yet approved.