function report( tag, loc ) {
		var msg = '"'+tag+'" is not one of the allowed tags ("'+opts.tags.join( '","' )+'")';
		context.report({
			'node': null,
			'message': msg,
			'loc': loc
		});
	}
Reports the error message.

@private
@param {string} tag - name of unknown tag
@param {Object} loc - lines of code (object with `start` and `end` properties)