proxy: go get should return different errors for "not found" vs "on blacklist"

[bketelsen]

proxy should return a not found error for a package that isn't found
the error for a package that is on the blacklist should be different from the "not found" error, to allow the go client to discriminate properly between non-existing packages and packages prohibited by proxy policy.

[arschles]

Agree. Personally I think we should make 403 Forbidden the standard for anything on the exclude list in the proxy

[marwan-at-work]

We should close this issue since we don't return 404s anymore. However, for better performance, we should open a new issue titled "return 404 and asynchronously fill cache" once cmd/go resolves this issue: golang/go#26334

[michalpristas]

Closing as blacklist returns StatusForbidden