return Err when image but no image backend

[o2sh]

I removed the panic!("No image backend found") from Info.fmt() and I added a check in case the user chose an image-backend not supported by his terminal.

---

By the way, I noticed that kitty::KittyBackend::supported() and sixel::SixelBackend::supported() need to println!() a sequence of characters in order to check if the corresponding backend is supported, resulting in two empty lines if both check fail:

I tried wrapping both methods into smth like (+ replace println! by print!):

    pub fn supported() -> bool {
        let is_supported = KittyBackend::is_supported();
        print!("\r");
        is_supported
    }

but it broke everything 😭