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

[Erik Joelsson]

Hello,

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/

/Erik