clearer error if endpoint missing DeviceAuthURL

deviceauth.go

@@ -74,6 +74,10 @@ func (c *DeviceAuthResponse) UnmarshalJSON(data []byte) error {
 // DeviceAuth returns a device auth struct which contains a device code
 // and authorization information provided for users to enter on another device.
 func (c *Config) DeviceAuth(ctx context.Context, opts ...AuthCodeOption) (*DeviceAuthResponse, error) {
+	if c.Endpoint.DeviceAuthURL == "" {
+		return nil, fmt.Errorf("endpoint missing DeviceAuthURL")
+	}
+
 	// https://datatracker.ietf.org/doc/html/rfc8628#section-3.1
 	v := url.Values{
 		"client_id": {c.ClientID},