public static HostRemovalFailure none() {
            return new HostRemovalFailure(Optional.empty(),
                                          Optional.empty(),
                                          new AllocationFailureReasonList(List.of()));
        }
Data class used for detailing why removing the given tenant from the given host was unsuccessful.
A failure might not be caused by failing to allocate a specific tenant, in which case the fields
will be empty.
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/maintenance/CapacityChecker.java