FD constraints could be more helpful in reporting errors

Something like:

    (A #= 0, B #= 2, C #= 4) #\/ (A #= 1, B #= 3, C #= 5)

using lib(fd) should not just silently fail, but should print an error
message of some kind.  (This happens if any non-arithmetic functor is
encountered in an FD constraint: e.g. `f(X) #=> g(Y)' and `[] #>= 3'.)

Cheers,
Warwick