Raise an error if WG1220 is not responding

wg1220.py

@@ -37,6 +37,10 @@ class Generator(object):
 
         self.conn.write(message)
         result = self.conn.read(7)
+
+        if len(result) == 0:
+            raise IOError("No result returned")
+
         log.debug("recv {}".format(fmt(result)))
 
         if checksum(result[1:6]) != ord(result[6]):