Added exceptions to gpio class

lib/Gpio.php

@@ -19,12 +19,24 @@
 
 namespace NoccyLabs\Gpio;
 
+use NoccyLabs\Gpio\Exception\HardwareException;
+
 class Gpio implements \ArrayAccess
 {
     protected $gpio = array();
     
     /** @var NoccyLabs\Gpio\GpioMapperInterface */
     protected $mapper;
+    
+    public function __construct()
+    {
+        if (!file_exists("/sys/gpio")) {
+            throw new HardwareException("gpio sysfs is not available");
+        }
+        if (!is_writable("/sys/gpio/export")) {
+            throw new HardwareException("gpio sysfs is not writable");
+        }
+    }
 
     public function refresh()
     {