Fix error when the device class dir doesn't exist

Author: WasabiFan

ev3dev/core.py

@@ -57,6 +57,10 @@
 
 # -----------------------------------------------------------------------------
 def list_device_names(class_path, name_pattern, **kwargs):
+
+    if not os.path.isdir(class_path):
+        return
+    
     """
     This is a generator function that lists names of all devices matching the
     provided parameters.

tests/api_tests.py

@@ -36,6 +36,9 @@ def test_device(self):
         d = ev3.Device('lego-sensor', 'sensor*')
         self.assertTrue(d.connected)
 
+        d = ev3.Device('this-does-not-exist')
+        self.assertFalse(d.connected)
+
     def test_medium_motor(self):
         def dummy(self):
             pass