libprocessgroup: Convert incorrect PLOGs to LOGs

PLOG depends on errno being set to a useful value, otherwise it will
print a meaningless error string. A few PLOG call sites occur where
either errno is not set at all, or it is set only some of the time where
there are already PLOG calls closer to where the error occurs. Convert
these PLOG calls to LOG.

Bug: 301871933
Change-Id: Ifa6bd2401f9dd9b84b2506e886336e89bac81bb1