Disallow box_approximation for unbounded sets

[schillic]

julia> X = Line([0.0, 1.0], [-1.0, 1.0]);

julia> box_approximation(X)  # master
Hyperrectangle{Float64, Vector{Float64}, Vector{Float64}}([NaN, NaN], [Inf, Inf])

julia> box_approximation(X)  # this branch
ERROR: ArgumentError: the box approximation of an unbounded set is undefined

[mforets]

makes sense