Nicer error message when `ATTUNE_API_ENDPOINT` is invalid

[jssblck]

Today, if you run attune with a ATTUNE_API_ENDPOINT value that specifies a server that doesn't exist, it results in a very unhelpful error:

ATTUNE_API_ENDPOINT=https://somehost.doesnotexist attune apt repo list 

thread 'main' panicked at packages/attune/src/bin/attune/main.rs:78:10:
Could not reach API server: reqwest::Error { kind: Request, url: "https://somehost.doesnotexist/api/v0/compatibility", source: hyper_util::client::legacy::Error(Connect, ConnectError("dns error", Custom { kind: Uncategorized, error: "failed to lookup address information: nodename nor servname provided, or not known" })) }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

We should capture errors like this and surface a nicer message for users.