RFR: JDK-8141696 Improve COMPARE_BUILD

[Tim Bell]

Hello Magnus:

> The new COMPARE_BUILD feature has proven valuable to build system 
> development. However, after use it leaves the build system in a 
> different state than prior to use, which requires manual intervention 
> for multiple runs.
>
> This patch fixes so that the build system is restored to the starting 
> point (except for unhandled crashes, e.g. in configure). It also moves 
> some error handling logic to InitSupport.gmk so it does not clutter 
> Init.gmk.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8141696
> WebRev: 
> http://cr.openjdk.java.net/~ihse/JDK-8141696-improve-COMPARE_BUILD/webrev.01

Looks good to me...

/Tim