libprocessgroup: Improve error messages

Change two PLOG() statements into LOG() statements since PLOG() should
only be used if errno has been set. Make it easier to find the code that
logs an error message.

Bug: 213617178
Signed-off-by: Bart Van Assche <bvanassche@google.com>
Change-Id: I73443f3adb7d7ba3fc0d39a55777f0b132529fbd