CR: Register and trigger custom wp_action upon token failure

[ramonschriks]

PR: Adding a custom hook upon token exchange failure creates to ability to add custom logic (such as redirecting to a custom unauthorized/forbidden page).

Moved the default logic when token exchange fails (redirect to '/'), to default action hook, and added custom action in registry so it gets registered.

Within WordPress, dev's can opt-in to this action so behaviour like this can be done whenever the token exchange fails (in our case we want to force a logout at auth0):

add_action('auth0_token_exchange_failed', function ( $throwable ) {
	do_action( 'auth0_logout' );
   }, 1
);

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[evansims]

Thanks for your contribution!