list-append's Evergreen report generator masks failures if a single iteration succeeds

The list-append workload, unlike all other jepsen workloads, runs 30 times (as opposed to 1). jepsen_report.sh erroneously flags a single test's success as success for all 30 tests.

This task shows a test that had a failure masked by this bug.

Fix jepsen_report.sh so that one failure causes the whole task to fail.