Option to disable host compiler fallback

[mcandre]

Checklist

• I've looked through the issues and pull requests for similar request
• This feature could be solved with a custom docker image (optional)

Describe your request

As a DevOp, I want a CLI flag that disables host compiler fallback in the event that a Docker image is not found, in order to reduce mistakes during complex build pipelines.

Current behavior:

$ cross build
[cross] warning: `cross` does not provide a Docker image for target aarch64-apple-darwin, specify a custom image in `Cross.toml`.
[cross] note: Falling back to `cargo` on the host.
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s

Instead of [cross] note: Falling back to cargo on the host., I want to see an error message.

Instead of continuing the build, I want cross to terminate immediately.

Instead of a successful exit code, I want a failure exit code emitted.

Describe why this would be a good inclusion for cross

The fallback behavior can result in accidents, where engineers think they are building a cross-compiling artifacts for one platform, and end up generates artifacts that target the host platform instead.

I want no fallback. I want the build to fail early, and loudly.

[Emilgardis]

This is a duplicate of #659 ! which is in the pipeline to be implemented for the next release (#661)!