fail when download asset does not contain the redirect URL

[jtarchie]

We've had issues that the get of the resource caches an incorrect value for the assets.

For example:

$ echo '{"source":{"user":"pivotal-cf-experimental","repository":"concourse-windows-worker-release"}, "version": {"tag":"3.3.3"}}' | /opt/resource/in /tmp
$ cat /tmp/resource.tgz
{"url":"https://api.github.com/repos/pivotal-cf-experimental/concourse-windows-worker-release/releases/assets/4426005" ... }

We were expecting the release.tgz to have been a large tarball. It appears that the Github API for assets returns a location. When we don't receive a location, we return the contents of the API call, as seen above.

We need a better error message.