cray-pals: improve eventlog error messages

Problem: cray_pals.c provides no indication of whether it has
received any events at all before it times out.

Improve the error message to say what the last event received was,
and whether any events have been received at all.

src/shell/plugins/cray_pals.c

@@ -496,7 +496,7 @@ static int read_future (flux_future_t *fut,
     json_t *o = NULL;
     json_t *context = NULL;
     json_t *array;
-    const char *name, *event = NULL;
+    const char *name = "<no events received>", *event = NULL;
     size_t index = 0;
     json_t *value;
     json_int_t portnum;
@@ -566,7 +566,8 @@ static int read_future (flux_future_t *fut,
             json_decref (o);
         }
     }
-    shell_log_error ("Timed out waiting for start event");
+    shell_log_error ("Timed out waiting for start event, last event received was %s",
+                     name);
     return -1;
 }