VAX: Allow SET CPU IDLE command to not specify a stability value.

Report invalid stability values and explain why they're invalid.

Fixes #281

VAX/vax_cpu.c

@@ -3719,8 +3719,8 @@ fprintf (st, "detection is operating system specific.  If idle detection is enab
 fprintf (st, "an incorrect operating system setting, simulator performance or correct\n");
 fprintf (st, "functionality could be impacted.  The default operating system setting is\n");
 fprintf (st, "VMS.  The value 'n', if present in the \"SET CPU IDLE={OS}:n\" command,\n");
-fprintf (st, "indicated the number of seconds which the simulator must run before idling\n");
-fprintf (st, "(and clock calibration) starts.\n\n");
+fprintf (st, "indicats the number of seconds which the simulator must run before idling\n");
+fprintf (st, "starts.\n\n");
 fprintf (st, "The CPU can maintain a history of the most recently executed instructions.\n");
 fprintf (st, "This is controlled by the SET CPU HISTORY and SHOW CPU HISTORY commands:\n\n");
 fprintf (st, "   sim> SET CPU HISTORY                 clear history buffer\n");

sim_timer.c

@@ -1004,18 +1004,14 @@ t_stat sim_set_idle (UNIT *uptr, int32 val, char *cptr, void *desc)
 t_stat r;
 uint32 v;
 
-if (sim_idle_rate_ms == 0) {
-    sim_printf ("Idling is not available, Minimum OS sleep time is %dms\n", sim_os_sleep_min_ms);
-    return SCPE_NOFNC;
-    }
-if ((val != 0) && (sim_idle_rate_ms > (uint32) val)) {
-    sim_printf ("Idling is not available, Minimum OS sleep time is %dms, Requied minimum OS sleep is %dms\n", sim_os_sleep_min_ms, val);
-    return SCPE_NOFNC;
-    }
-if (cptr) {
+if (sim_idle_rate_ms == 0)
+    return sim_messagef (SCPE_NOFNC, "Idling is not available, Minimum OS sleep time is %dms\n", sim_os_sleep_min_ms);
+if ((val != 0) && (sim_idle_rate_ms > (uint32) val))
+        return sim_messagef (SCPE_NOFNC, "Idling is not available, Minimum OS sleep time is %dms, Requied minimum OS sleep is %dms\n", sim_os_sleep_min_ms, val);
+if (cptr && *cptr) {
     v = (uint32) get_uint (cptr, 10, SIM_IDLE_STMAX, &r);
     if ((r != SCPE_OK) || (v < SIM_IDLE_STMIN))
-        return SCPE_ARG;
+        return sim_messagef (SCPE_ARG, "Invalid Stability value: %s.  Valid values range from %d to %d.\n", cptr, SIM_IDLE_STMIN, SIM_IDLE_STMAX);
     sim_idle_stable = v;
     }
 sim_idle_enab = TRUE;