installer: use mysqli_error when using mysqli (closes #696)

install/index.php

@@ -191,7 +191,7 @@
 			if (!$result) {
 				$query = htmlspecialchars($query);
 				if ($die_on_error) {
-					die("Query <i>$query</i> failed: " . ($link ? mysql_error($link) : "No connection"));
+					die("Query <i>$query</i> failed: " . ($link ? function_exists("mysqli_connect") ? mysqli_error($link) : mysql_error($link) : "No connection"));
 				}
 			}
 			return $result;