MokManager: handle the error status from ReadKeyStroke

On some machines, even though the key event was signaled, ReadKeyStroke
still got EFI_NOT_READY. This commit handles the error status to avoid
console_get_keystroke from returning unexpected keys.

Signed-off-by: Gary Ching-Pang Lin <glin@suse.com>

Conflicts:
	MokManager.c