Throw friendly error when device is disconnected (#373)

* Throw friendly error when device is disconnected

* Update error code

Author: WasabiFan

ev3dev/core.py

@@ -223,7 +223,7 @@ def _attribute_file_open( self, name ):
     def _get_attribute(self, attribute, name):
         """Device attribute getter"""
         if self.connected:
-            if None == attribute:
+            if attribute is None:
                 attribute = self._attribute_file_open( name )
             else:
                 attribute.seek(0)
@@ -235,12 +235,12 @@ def _get_attribute(self, attribute, name):
     def _set_attribute(self, attribute, name, value):
         """Device attribute setter"""
         if self.connected:
-            if None == attribute:
-                attribute = self._attribute_file_open( name )
-            else:
-                attribute.seek(0)
-
             try:
+                if attribute is None:
+                    attribute = self._attribute_file_open( name )
+                else:
+                    attribute.seek(0)
+
                 attribute.write(value.encode())
                 attribute.flush()
             except Exception as ex:
@@ -263,6 +263,11 @@ def _raise_friendly_access_error(self, driver_error, attribute):
                 else:
                     raise ValueError("The given speed value was out of range. Max speed: +/-" + str(max_speed)) from driver_error
             raise ValueError("One or more arguments were out of range or invalid") from driver_error
+        elif driver_error.errno == errno.ENODEV or driver_error.errno == errno.ENOENT:
+            # We will assume that a file-not-found error is the result of a disconnected device
+            # rather than a library error. If that isn't the case, at a minimum the underlying
+            # error info will be printed for debugging.
+            raise Exception("%s is no longer connected" % self) from driver_error
         raise driver_error
 
     def get_attr_int(self, attribute, name):