fix(launcher): ignore exit code when killing/timeouting

Most of the browsers returns "not-normally" (because of receiving a signal) and therefore exit event gets null exit code. But PhantomJS exits "normally" with status code 1 (after receiving TERM signal).

I think this logic is better - show error, if process exits during a state which does not expect exiting (CAPTURED, BEING_CAPTURED).

Closes #444