sleep: reduce double logging and improve messages and comments a bit

sleep: reduce double logging and improve messages and comments a bit

read_battery_capacity_percentage() was already logging, but with a slightly
different wording.

More could be done, I just touched the most noticable places. Especially
in debug messages, it is much more useful to be direct about what couldn't
be accessed or parsed, instead of providing "descriptive names" which are
not useful to the user at all, who then needs to read the code to figure out
what was the actual property name.