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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[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.

[joedolson]

This should be #custom-url-error.

[joedolson]

I know I suggested this text, but given the flexibility in what the possible values can be, probably need to make this more general yet. Perhaps "Content is required for the link's href attribute."