lib/signals: respect value of exitErrorIsOk and set this to false where suitable
lib/signals: respect value of exitErrorIsOk and set this to false where suitable

Also, fix TestHandlerCustomSignal to issue close instead of stop at the
end -- otherwise, the child will exit immediately with a double signal
instead of cleanly as the test intended.

Change-Id: I8738e502306af01c9dac7ce5b215bb0c614e8a94

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref