We now produce a friendlier error when we can't load a Space
SpaceState.prop() now return an Option

This has always been a design bug.  Decided that we need to stop
throwing exceptions and handle this correctly.
Fixed the Space-Prop-Reference bug

It had been that, if the Space Thing itself used a Property defined *in*
that Space, it would crash at load time.  Previous checkins prevented
the hard crash; this now actually gets it working right.
Circular Property references now load successfully

Although, for some reason, they don't seem to *display* right, so
something may yet be wrong...