Only allow 5 consecutive missed guidestars.

I was too aggressive with my previous fix to SEP MultiStar, where I allowed missing guide stars when other reference stars were detected. I found occasions where this was done for many many consecutive frames. It was only intended to be a temporary robustness, e.g. for a cloud passing quickly. Therefore this change only allows this behavior for 5 consecutive guider iterations. More than that and it reverts to the previous lost-guide-star behavior.