jwt: use RetrieveError for invalid status code errors

CL 84156 added oauth2.RetrieveError to the oauth2 and clientcredentials
packages, but missed using it in jwt.

Change-Id: I06d77cd18667526bfc869ebc1b5cc2bcbabc03a6
Reviewed-on: https://go-review.googlesource.com/85457
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

jwt/jwt.go

@@ -124,7 +124,10 @@ func (js jwtSource) Token() (*oauth2.Token, error) {
 		return nil, fmt.Errorf("oauth2: cannot fetch token: %v", err)
 	}
 	if c := resp.StatusCode; c < 200 || c > 299 {
-		return nil, fmt.Errorf("oauth2: cannot fetch token: %v\nResponse: %s", resp.Status, body)
+		return nil, &oauth2.RetrieveError{
+			Response: resp,
+			Body:     body,
+		}
 	}
 	// tokenRes is the JSON response body.
 	var tokenRes struct {