Better error message for set inclusion usage

[mforets]

Similar in spirit to #1282, we can add a better error message to the (incorrect) usage:

julia> Z = rand(Zonotope);

julia> center(Z) ⊆ Z  # explain saying why this is wrong and show the correct usage
ERROR: MethodError: no method matching length(::Zonotope{Float64})
Closest candidates are:
  length(::Core.SimpleVector) at essentials.jl:561
  length(::Base.MethodList) at reflection.jl:801
  length(::Core.MethodTable) at reflection.jl:875
  ...
Stacktrace:
 [1] issubset(::Array{Float64,1}, ::Zonotope{Float64}) at ./abstractset.jl:234
 [2] top-level scope at none:0