Ensure that the #nodes in the job map gets properly updated when using the sequential mapper. Provide some further diagnostic info to help understand the problem when encountered.

orte/mca/rmaps/base/help-orte-rmaps-base.txt

@@ -97,6 +97,11 @@ Your job failed to map. Either no mapper was available, or none
 of the available mappers was able to perform the requested
 mapping operation. This can happen if you request a map type
 (e.g., loadbalance) and the corresponding mapper was not built.
+
+  Mapper result:    %s
+  #procs mapped:    %d
+  #nodes assigned:  %d
+
 #
 [unrecognized-policy]
 The specified %s policy is not recognized:

orte/mca/rmaps/base/rmaps_base_map_job.c

@@ -304,7 +304,9 @@ void orte_rmaps_base_map_job(int fd, short args, void *cbdata)
      * the map, then that's an error
      */
     if (!did_map || 0 == jdata->num_procs || 0 == jdata->map->num_nodes) {
-        orte_show_help("help-orte-rmaps-base.txt", "failed-map", true);
+        orte_show_help("help-orte-rmaps-base.txt", "failed-map", true,
+                       did_map ? "mapped" : "unmapped",
+                       jdata->num_procs, jdata->map->num_nodes);
         ORTE_ACTIVATE_JOB_STATE(jdata, ORTE_JOB_STATE_MAP_FAILED);
         OBJ_RELEASE(caddy);
         return;

orte/mca/rmaps/seq/rmaps_seq.c

@@ -201,6 +201,13 @@ static int orte_rmaps_seq_map(orte_job_t *jdata)
     }
 #endif
 
+    /* initialize all the nodes as not included in this job map */
+    for (j=0; j < orte_node_pool->size; j++) {
+        if (NULL != (node = (orte_node_t*)opal_pointer_array_get_item(orte_node_pool, j))) {
+            ORTE_FLAG_UNSET(node, ORTE_NODE_FLAG_MAPPED);
+        } 
+    }
+    
     /* cycle through the app_contexts, mapping them sequentially */
     for(i=0; i < jdata->apps->size; i++) {
         if (NULL == (app = (orte_app_context_t*)opal_pointer_array_get_item(jdata->apps, i))) {