Changing wording for child exit messages

When one of the child processes was exiting word "failed" has been used,
which might be a bit too scary, considering that process was terminated
intentionally. This patch replaces "fail" with "exit" which is more generic
and covers cases when this was an intentional exit, not an unexpected
failure.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>