Handle internal display being powered down

Can't detect refresh rate like this right now... but also this case is
reportedly dangerous too, so refusing to install is probably the right
call.

Signed-off-by: Hector Martin <[email protected]>

Author: marcan

src/system.py

@@ -183,4 +183,7 @@ def get_refresh_rate(self):
                 assert main_display is None
                 main_display = disp
 
+        if main_display is None:
+            return "(Unknown or off)"
+
         return main_display["_spdisplays_resolution"].split()[-1]