detect failed directories in /pkg/dep/resources

[marwan-at-work]

1. What version of Go are you using (go version)?
   1.8.1

2. What operating system and processor architecture are you using?
   darwin x86

3. What did you do?
   Say a dep init process gets interrupted (ctrl+c), the underlying cache in $GOPATH/pkg/dep/resources could be in a false state because the dependency that was being copied in to the cache could either be empty or incomplete.
   Therefore, the next time you run dep init, if that same dependency is needed, the cache will cause the entire process to error out.

4. What did you expect to see?
   dep init working.

5. What did you see instead?

$ dep init
Searching GOPATH for projects...
  Using master as constraint for direct dep github.com/golang/protobuf
  Locking in master (fec3b39) for direct dep github.com/golang/protobuf
  Using master as constraint for direct dep golang.org/x/net
  Locking in master (5602c73) for direct dep golang.org/x/net
  Using master as constraint for direct dep google.golang.org/grpc
  Locking in master (1c69e4c) for direct dep google.golang.org/grpc
  Locking in master (470f45b) for transitive dep golang.org/x/text
Unable to cache golang.org/x/text
  Locking in master (411e09b) for transitive dep google.golang.org/genproto
Using network for remaining projects...
Unable to retrieve local repo information:

[marwan-at-work]

@sdboyer feel free to assign me, i'll take a crack at it after work and probably bother you a bunch of times on slack lol. thanks!

[sdboyer]

duly assigned :)

[carolynvs]

Fixed by #596