Some fields that were never empty in our original test image
can apparently be empty in images that one finds in the wild. This
adds appropriate checking so that blank file lists and missing
image IDs no longer cause crashes.
* Fix two small compiler errors
Apparently, in go, `fmt.Prntln("\n")` is a build error - you can't
have a redundant newline there. Also, we had the wrong format string
somewhere.
* Use $GOPATH to find dep
When I first pulled this branch, all the tests failed because,
although `dep` was being installed to my local machine, it wasn't
being added to $PATH. Rather than mess with $PATH, I decided to
add $GOPATH/bin to the beginning of the invocation.
