Previously, error message only showed "<nil>" when child and pid were the
same (since no error is returned by the Wait4 syscall in this case) which
occurs when the process has incorrectly terminated. A new error message
was added to improve clarity for such a case. Tests for this function were
modified to reflect the improved distinction between process termination
and error.
PiperOrigin-RevId: 204018107
Change-Id: Ib38481c9590405e5bafcb6efe27fd49b3948910c
Brielle Broder
Fabricio Voznika