provide a better message when a package needs to be built (and deps/deps.jl is missing)

[Kevin Cantú]

I filed JuliaLang/julia#8815, which they closed, but it seems that we can fix it with Nettle, like so:

if isfile(joinpath(Pkg.dir("Nettle"),"deps","deps.jl"))
   include("../deps/deps.jl")
else
    error("Nettl not properly installed. Please run Pkg.build(\"Nettle\")")
end

The relevant BinDeps issue: JuliaPackaging/BinDeps.jl#100

[Elliot Saba]

Closed by 97268c4

[Kevin Cantú]

Cool!