After https://bugs.openjdk.java.net/browse/JDK-8209765 was merged the Appveyor build no longer fails if a test fails. This is because the powershell script executes successfully, and returns a 0 exit code, even though the last exit code from invoking gradle was non-zero.

To fix this we manually check the exit code if we are on Appveyor, and exit with it. This should let the build fail as intended.