Consider HTTP status for more meaningful errors

The krew CI has quite a few problems on travis, supposedly due to
API quota violation. For more meaningful errors, we now consider the
HTTP status code of the GitHub API response.

cmd/krew/cmd/internal/fetch_tag.go

@@ -37,6 +37,9 @@ func FetchLatestTag() (string, error) {
 		return "", errors.Wrapf(err, "could not GET the latest release")
 	}
 	defer response.Body.Close()
+	if response.StatusCode != http.StatusOK {
+		return "", errors.Errorf("expected HTTP status 200 OK, got %s", response.Status)
+	}
 
 	var res struct {
 		Tag string `json:"tag_name"`