Prevent error message on svc power reboot

With the changes to sys.powerctl this variable is no longer set on a
power event - the power action is simply carried out. (This is to avoid
possible DoS of these power functions.) However, this means that this
property can no longer be read to see if a power event is happening.
Since there is no longer a way of telling if a remote exception is real
or not, best not to print out an error.

Note that this bug occurs on some devices, not all. I was able to
reproduce it reliably on a Pixel 7a, but not on a Pixel 7 or Cuttlefish.

Bug: 318323013
Test: adb shell svc power reboot

Google: 2983091
Change-Id: Icbc1ec1b7882140d4d742b056ea2adaafae67ee9