fetch can cache an error page for a plugin download

[asottile-sentry]

I'm using the tool built from 085ab00

apologies I don't have too much more details than this --

$ gocd configrepo --yaml fetch
Downloading https://github.com/tomzo/gocd-yaml-config-plugin/releases/download/0.14.1/yaml-config-plugin-0.14.1.jar                              
  Fetched 212 B/212 B (100.0%) complete
$ echo $?
0

the 212B response is an error from github

I think it's because the status code is not checked here?

gocd-cli/utils/http.go

Line 30 in 55cf87f

err = dub.New().Get(url).Do(func(res *dub.Response) (err error) {

later invocations will fail with something like:

Could not find plugin "yaml.config.plugin" in your plugin path.
	Attempting to download plugin "yaml.config.plugin"...
zip: not a valid zip file