RFR: JDK-8204564: Need better error output when GenerateLinkOptData fails

[Tim Bell]

Erik:
> Some recent build failures reminded me of this issue and increased the
> need to fix it ASAP to be able to diagnose them. GenerateLinkOptData.gmk
> runs a java tool to generate data which is later used to optimize the
> JDK image with jlink. The problem here is that the tool needs to print
> some of that data on stdout, which the build captures as one build
> artifact, and will print some "garbage" on stderr if things go well. So
> currently we just hide all that output from the build log. Because of
> this, if something goes wrong, we won't see the error.
>
> This patch tries to capture the stderr output and only print it (along
> with stdout) if the java command fails.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8204564
>
> Webrev: http://cr.openjdk.java.net/~erikj/8204564/webrev.01/

Looks good.

Tim