Tonight Jeff B and Jeff K struggled with ALS DIFF for a while.  The problem was with the way the error conditions are handled in the guardian, I've removed the check of the diff beatnote for now but another day we should think carefully about how we want these checks to be handled. 

The ALS_DIFF beatnote was less than -15dB, which is a fault condition for the DIFF guardian, which caused it to make a jump to DOWN.  This caused the ISC lock guardian to recognize a lockloss.  This is an old bug, which has probably been around for more than 2 years. 

For now I have removed the beatnote check, but there are other checks which have the same error handling, so we should come back to looking at how this is done.