trezor/kk: when using old fw, wizard did not display instructions properly

electrum/plugins/keepkey/keepkey.py

@@ -141,7 +141,10 @@ class KeepKeyPlugin(HW_PluginBase):
                      'download the updated firmware from {}')
                    .format(self.device, client.label(), self.firmware_URL))
             self.print_error(msg)
-            handler.show_error(msg)
+            if handler:
+                handler.show_error(msg)
+            else:
+                raise Exception(msg)
             return None
 
         return client

electrum/plugins/trezor/trezor.py

@@ -157,7 +157,10 @@ class TrezorPlugin(HW_PluginBase):
                      'download the updated firmware from {}')
                    .format(self.device, client.label(), self.firmware_URL))
             self.print_error(msg)
-            handler.show_error(msg)
+            if handler:
+                handler.show_error(msg)
+            else:
+                raise Exception(msg)
             return None
 
         return client