RFR(xxs): 8213017: jspawnhelper: need to handle pipe write failure when sending return code

[Thomas Stüfe]

May I please have reviews for this trivial patch?

On Linux, I get compiler warnings when building the jspawnhelper. It
complains about not checking the return code to write when
communicating an error code back to be parent.

I decided to just fix it since this is indeed a bug. But there is no
real way to produce error output since stdout/err are redirected. So I
decided to just exit with a clear error code instead.

bug: https://bugs.openjdk.java.net/browse/JDK-8213017
cr: http://cr.openjdk.java.net/~stuefe/webrevs/8213017-jspawnhelper-write-return-
code/webrev_0/webrev/

Thanks, Thomas