error/panic on fallback to host cargo or when in CI environment

[Emilgardis]

cross/src/main.rs

Lines 344 to 350 in eaeff5b

	let image_exists = match docker::image(&config, &target) {
	Ok(_) => true,
	Err(err) => {
	eprintln!("Warning: {} Falling back to `cargo` on the host.", err);
	false
	}
	};

[jpmckinney]

I would also like an option to error/panic on fallback. This is especially useful in CI. I guess for now I can try to capture stderr and the exit code (edit: after trying a few different things, I haven't found a way for GitHub Actions output to be informative while still capturing some streams).

[Emilgardis]

thats something I hadn't considered, maybe providing a way to not allow any warnings would be helpful.

We definitely want this included in the next release