init: fix errno handling on KillProcessGroup

KillProcessGroup can return -1 without errno set, which produces error
messages like this:

init: Sending signal 15 to service 'vold' (pid 1806) process group...
init: failed to kill 2 processes for processgroup 1806
init: killProcessGroup(0, 1806, 15) failed: Success

Test: boot and reboot hikey.

Change-Id: I51f242da234daee84795a3fbdffe9ad1f4567140
Signed-off-by: William Roberts <william.c.roberts@intel.com>