handle missing battery case
handle missing battery case

I run the same bumblebee-status configuration on my laptop and my
workstation. On my laptop, the upower module works fine: it says "ac"
when plugged in, charging, all that stuff is great.

But on my workstation, it's completely broken: it thinks there's a
battery (which is a mistake: there is no battery at all, apart maybe
from the CMOS battery, but that's not covered by upower), and it
thinks it's discharged, which makes a very noisy warning in the bar.

Now maybe there's something wrong with dbus, Debian, the kernel,
Linux, or some thing else in the stack. All I know is that
`self.power.get_display_device()` returns something like a valid
dbus object here and from there it confuses the heck out of the
module.

So this just adds a function to check if the actual device we're
talking about is actually present, and bails earlier otherwise.

Before: battery logo and "0% 00:00m!", all marked as critical ("red")

After: "ac" with the plugged in logo, not marked critical ("black")

tobi-wan-kenobi/bumblebee-status,tobi-wan-kenobi/bumblebee-status