Be [slightly] more descriptive. :-)

This commit was SVN r7198.

orte/tools/orterun/help-orterun.txt

@@ -87,4 +87,6 @@ This means that %s exited before it received notification that all
 started processes had terminated.  You should double check and ensure
 that there are no runaway processes still executing.
 [orterun:empty-prefix]
-Specified an empty prefix (only containing slashes).
+A prefix was supplied to %s that only contained slashes.  
+
+This is a fatal error; %s will now abort.  No processes were launched.

orte/tools/orterun/orterun.c

@@ -1071,7 +1071,7 @@ static int create_app(int argc, char* argv[], orte_app_context_t **app_ptr,
             param_len--;
             if (0 == param_len) {
                 opal_show_help("help-orterun.txt", "orterun:empty-prefix",
-                                true);
+                                true, orterun_basename, orterun_basename);
                 return ORTE_ERR_FATAL;
             }
         }