Bugfix: don't panic if user hits non-active key.

diff --git a/qwerty.py b/qwerty.py
index e9ba5eb..5590e8a 100755
--- a/qwerty.py
+++ b/qwerty.py
@@ -545,7 +545,10 @@ class GUI(object):
         os.system(cmd + " &")

     def do_button(self,key):
-        self.on_button(self.buttons[key.upper()])
+        try:
+            self.on_button(self.buttons[key.upper()])
+        except KeyError:
+            return

     def send_event(self,win,ctype,data):
         data = (data+[0]*(5-len(data)))[:5]