Find a way to let Appveyor know the build failed from exit code or the build.log

[GabLeRoux]

As shown in the following pipeline:
https://ci.appveyor.com/project/GabLeRoux/unity3d-encrypted-appveyor-poc/builds/26298852

The build failed, but appveyor says it's a success. Here's how I do this with bash:

UNITY_EXIT_CODE=$?

if [ $UNITY_EXIT_CODE -eq 0 ]; then
  echo "Run succeeded, no failures occurred";
elif [ $UNITY_EXIT_CODE -eq 2 ]; then
  echo "Run succeeded, some tests failed";
elif [ $UNITY_EXIT_CODE -eq 3 ]; then
  echo "Run failure (other failure)";
else
  echo "Unexpected exit code $UNITY_EXIT_CODE";
fi

source: https://gitlab.com/gableroux/unity3d-gitlab-ci-example/blob/d40a17e7/ci/build.sh