Fix confusing error when unpack fails due to network error
Fix confusing error when unpack fails due to network error

Before: `Error: .`
After: `Error: Failed downloading http://bad_url`

robooo/luarocks,robooo/luarocks,luarocks/luarocks,usstwxy/luarocks,aryajur/luarocks,xpol/luarocks,robooo/luarocks,aryajur/luarocks,coderstudy/luarocks,aryajur/luarocks,tst2005/luarocks,xiaq/luarocks,xpol/luavm,coderstudy/luarocks,xpol/luarocks,usstwxy/luarocks,tarantool/luarocks,lxbgit/luarocks,usstwxy/luarocks,xpol/luarocks,rrthomas/luarocks,xiaq/luarocks,tst2005/luarocks,xpol/luainstaller,xpol/luainstaller,luarocks/luarocks,ignacio/luarocks,coderstudy/luarocks,tst2005/luarocks,xpol/luainstaller,tarantool/luarocks,starius/luarocks,lxbgit/luarocks,lxbgit/luarocks,starius/luarocks,xiaq/luarocks,xpol/luavm,keplerproject/luarocks,coderstudy/luarocks,keplerproject/luarocks,xpol/luavm,robooo/luarocks,aryajur/luarocks,tarantool/luarocks,ignacio/luarocks,rrthomas/luarocks,xiaq/luarocks,starius/luarocks,lxbgit/luarocks,xpol/luavm,rrthomas/luarocks,rrthomas/luarocks,luarocks/luarocks,usstwxy/luarocks,ignacio/luarocks,ignacio/luarocks,keplerproject/luarocks,xpol/luavm,keplerproject/luarocks,xpol/luainstaller,tst2005/luarocks,xpol/luarocks,starius/luarocks