double Pkg.add error message

[vtjnash]

julia> Pkg.add("IUP")
ERROR (unhandled task failure): unknown package IUP
 in error at error.jl:19
 in anonymous at task.jl:47
ERROR: unknown package IUP
 in wait at task.jl:57
 in sync_end at ./task.jl:331
 in add at pkg/entry.jl:339
 in add at pkg/entry.jl:71
 in anonymous at pkg/dir.jl:28
 in cd at ./file.jl:20
 in cd at pkg/dir.jl:28
 in add at pkg.jl:21

on 6bf81b3

[JeffBezanson]

A fine example of the inherent race condition in #10405.

[kshyatt]

Tested on latest master and this seems to have been resolved.