Handle error in Godep strip WalkFunc - #990

@mattfarina - this is similar to your fix #953 - in the godep code path. Can we get this in? Hitting this with Go 1.10

While this patch works around the crash and is correct anyway, the real fix is to return filepath.SkipDir rather than nil if we call os.RemoveAll() later in the function (like in #956, although that also included a bunch of other refactoring)

Can this fix (or an equivalent fix) please be merged, and a new release tagged? This is impacting distributors like Homebrew who ship go software.

https://github.com/Masterminds/glide/releases/tag/v0.13.2 was released yesterday with a bugfix for this particular issue. This stack trace in particular was fixed for Helm, so it's likely fixed for other projects. This PR may be defunct in that case and should probably be closed.

0.13.2 includes #956, which fixes the crash when skipping vendor/ but not when skipping Godeps/_workspace/

#1033 has the correct fix. This should be closed.

Closing in favor of #1033