Adopt systemd error reporting mechanism

[engelmi]

We also shoud probably pick up the systemd error reporting mechanism of returning -errno style integers instead of just true/false for "success"/"error". That way callers can get at least some more nformation on exactly what failed. In many cases we already call into systemd APIs which can give this info. For example:

bool node_export(Node *node) {
        Manager *manager = node->manager;

        int r = sd_bus_add_object_vtable(
                        manager->user_dbus,
                        &node->export_slot,
                        node->object_path,
                        NODE_INTERFACE,
                        node_vtable,
                        node);
        if (r < 0) {
                fprintf(stderr, "Failed to add node vtable: %s\n", strerror(-r));
                return false;
        }

        return true;
}

This would probably be nicer if it was something like:

int node_export(Node *node) {
        Manager *manager = node->manager;

        int r = sd_bus_add_object_vtable(
                        manager->user_dbus,
                        &node->export_slot,
                        node->object_path,
                        NODE_INTERFACE,
                        node_vtable,
                        node);
        if (r < 0) {
                hirte_log(HIRTE_LOG_WARNING, "Failed to add node vtable: %s\n", strerror(-r));
        }

        return r;
}

[rhatdan]

SGTM