
I hope, above picture explains itself. There is a components name present (PART NAME 1 and PART NAME 2) if selection is made. If selection is not present, it is empty. If it is empty we know what side of constraint is missing. In case of failed constraint keep part name but add "Missing Reference". Also, selection colors, which were supposed to help us detect what is selected should be more distinguishable, like red and yellow, and not like these two shades of blue colors.
Below is the info on failed constraint.

We can see that info for selection exists: Part1 and Part2. This should be incorporated into Edit Constraint windows as I proposed. Also, add "Missing Reference" next to Part Name so we know what is missing and on what part.