Better script (system) failure tracking

[tcojean]

Gitlab runner has a number of error tracking features,

• Failures in the user's job script should raise a BUILD_FAILURE_EXIT_CODE, this should already always be the case
• Failures in the script itself, such as invalid output, connection issues, ... should raise a SYSTEM_FAILURE_EXIT_CODE, see https://docs.gitlab.com/runner/executors/custom.html#system-failure

The second one is currently missing and should be done more properly in the case where something goes wrong in the script itself.