codal_port/drv_display: Raise any pending exc's if animation stopped.

This wait_for_event() function should really be doing (pseudo-code):

    while (1) {
        try:
            mp_handle_pending(true);
            microbit_hal_idle();
        finally:
            async_stop()
    }

But for efficiency (not to use nlr_push) it doesn't call mp_handle_pending,
which could raise.  So this function must be called if there is an
exception, so that exception can be processed correctly.

Fixes issue #134.

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

src/codal_port/drv_display.c

@@ -67,6 +67,7 @@ STATIC void wait_for_event() {
         // allow CTRL-C to stop the animation
         if (MP_STATE_THREAD(mp_pending_exception) != MP_OBJ_NULL) {
             async_stop();
+            mp_handle_pending(true);
             return;
         }
         microbit_hal_idle();