Some battery drivers will remove their sysfs directory when the
battery goes from charging to discharging, or vice verse.
This caused yambar’s battery module to terminate, resulting in the
last known battery state to “freeze”.
With this patch, failure to read the battery directory the *first*
time is still considered a hard failure, resulting in an error message
and then termination.
However, subsequent failures, i.e. while polling the battery state,
is *not* considered fatal; we simply don’t update the bar, and retry
again the next poll interval. Error messages are still logged however.
Closes#44