devel_glfw3.1: Return errors

The first commit causes every GLFW function to return an error. The second commit streamlines the error handling process by making use of the new `fetchError` function in place of `<-lastError`.
Style nitpick, but I think the `select` looks better like this:

```Go
func fetchError() error {
	select {
	case err := <-lastError:
		return err
	default:
		return nil
	}
}
```