Prevent error in Menu location checkbox settings

[sabernhardt]

• Creates a text string to display for invalid menu ID.
• Assigns id and aria-describedby to the connect the span with the checkboxes.

Trac 37026

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[github-actions]

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[sabernhardt]

Before the patch, an invalid menu ID can result in a PHP error:

With the patch, the text says that the ID is invalid (but it is not an error message):

Note: To assign an invalid ID, with Twenty Twenty-One as the active theme, I needed to edit the theme_mods_twentytwentyone value in the database, in the *_options table.