mgadm should handle API errors gracefully

[rcgoodfellow]

There are several places like this in the mgadm code where we unwrap API calls like this.

maghemite/mgadm/src/bgp.rs

Line 584 in 2524913

let response = c.read_router(asn).await.unwrap();

This results in a crash and corefile when the API returns an error, even for something like a 400 error. While mgadm is just a local debugging tool, this is not ideal as it leaves core files around which show up in rack health checks.