Make the request for Directly's remote JavaScript.

@returns {Promise} Promise that resolves after the script loads or fails
function loadDirectlyScript() {
	return new Promise( ( resolve, reject ) => {
		loadScript( DIRECTLY_RTM_SCRIPT_URL, function( error ) {
			if ( error ) {
				return reject( new Error( `Failed to load script "${ error.src }".` ) );
			}
			resolve();
		} );
	} );
}