Clarify that an 'error' event may or may not be followed by an 'exit'
event and that it's not safe to make assumptions either way.
2. The process could not be killed, or
3. Sending a message to the child process failed for whatever reason.
+Note that the `exit`-event may or may not fire after an error has occured. If
+you are listening on both events to fire a function, remember to guard against
+calling your function twice.
+
See also [`ChildProcess#kill()`](#child_process_child_kill_signal) and
[`ChildProcess#send()`](#child_process_child_send_message_sendhandle).