How to handle shortcode variables that are incorrect

I am using building some custom shortcode that will create styled links to specific categories. However, I need to build a failsafe so that if the client puts a non-existent category name into the shortcode, things fail elegantly. Currently, they will use shortcode that looks like [add_category category=photographer-renosparks]

My function looks like

function add_category_function( $atts ) {
	if ($atts['category']) {
		$cat = $atts['category']; //Pull category name from shortcode variable in page
		$idObj = get_category_by_slug( $cat ); // Get the ID of the category (Includes Line Below)
		$category_id = $idObj->term_id;
		$cat_name = get_the_category_by_id( $category_id ); // Get the nice Name of the category based on it's ID
		$category_link = get_category_link( $category_id ); // Get the link to the category based on it's ID
	} else {
		$cat_name = "noThumb";
		$category_link = '#';
	}
	$wtf = get_bloginfo("template_url");
	$slideshow_output = '';
	$image = $wtf . '/images/categories/' . strtolower($cat_name) . '.jpg';
	$slideshow_output.= '<div class="vendor_category_block"><div class="vendor_category_photo"><img src="' . $image . '" /></div><div class="vendor_category_title"><a href="' . $category_link . '" title="' . $cat_name . '" class="vendor_category_link">' . $cat_name . '</a></div></div>'; //create the HTML for the link.
	return $slideshow_output;
 }
 function register_shortcodes(){
	add_shortcode('add_category', 'add_category_function');

Currently, if they create code that doesn’t contain a legit category slug, I get the following error:
“Catchable fatal error: Object of class WP_Error could not be converted to string in /homepages/46/d188029515/htdocs/mattandjentry_root/otgb/wp-content/themes/otgb_1/functions.php on line 177”

So there is a problem with my if statement. Clearly it’s not working. What would be a proper if statement to give this a failover?