cmd/flux-job: fix module not loaded error

Improve error handling in flux-job submitbench to
be more helpful if a job is submitted while the job-ingest
module is not loaded.

Also, avoid printing "(null)" when there is no textual
error message available in the future.

src/cmd/flux-job.c

@@ -210,9 +210,16 @@ void submitbench_continuation (flux_future_t *f, void *arg)
 {
     struct submitbench_ctx *ctx = arg;
     flux_jobid_t id;
-
-    if (flux_job_submit_get_id (f, &id) < 0)
-        log_err_exit ("%s", flux_future_error_string (f));
+    const char *errmsg;
+
+    if (flux_job_submit_get_id (f, &id) < 0) {
+        if (errno == ENOSYS)
+            log_msg_exit ("submit: job-ingest module is not loaded");
+        else if ((errmsg = flux_future_error_string (f)))
+            log_msg_exit ("submit: %s", errmsg);
+        else
+            log_err_exit ("submit");
+    }
     printf ("%llu\n", (unsigned long long)id);
     flux_future_destroy (f);