We shouldn't be allowed to set_indexing_prop! on nonexistent vertices

[sbromberger]

julia> g = MetaGraph(1)
{1, 0} undirected Int64 metagraph with Float64 weights defined by :weight (default weight 1.0)

julia> set_indexing_prop!(g, 2, :index, "v2")
Set(Symbol[:index])

julia> g.vprops
Dict{Int64,Dict{Symbol,Any}} with 2 entries:
  2 => Dict{Symbol,Any}(:index=>"v2")
  1 => Dict{Symbol,Any}(:index=>"index1")