Added the ability to deal with division without crashing.
Also threw exception instead of returning null if an unknown
operator is used, like AND, NOT, OR, etc.  Nulls should never
be returned as an expression value as long as user scripts cannot
have the capacity to check for a null.
Fixed vector division operations.  Also catch the case where any
unsupported operator might be used on a vector (it used to return null
in that case, which kos scripts don't currently have the ability
to check for.  Nulls should never end up in a user's variable so long
as kos scripts can't compare against null in an if check.)
sanity checks around binding