Tracker Checker: handle HTTP Tracker error `ConnectionRefused`

[josecelano]

Parent issue: https://github.com/torrust/torrust-tracker/issues/669 Depends on: https://github.com/torrust/torrust-tracker/issues/680

When you run the Tracker Checker with a HTTP Tracker service not running you get this error:

TORRUST_CHECKER_CONFIG='{
    "udp_trackers": [],
    "http_trackers": ["http://127.0.0.1:7070"],
    "health_checks": []
}' cargo run --bin tracker_checker

Running checks for trackers ...
UDP trackers ...
HTTP trackers ...
thread 'main' panicked at /home/josecelano/Documents/git/committer/me/github/torrust/torrust-tracker/src/shared/bit_torrent/tracker/http/client/mod.rs:87:61:
called `Result::unwrap()` on an `Err` value: reqwest::Error { kind: Request, url: Url { scheme: "http", cannot_be_a_base: false, username: "", password: None, host: Some(Ipv4(127.0.0.1)), port: Some(7070), path: "/announce", query: Some("info_hash=%9C8B%22%13%E3%0B%FF%21%2B0%C3%60%D2o%9A%02%13d%22&peer_addr=192.168.1.88&downloaded=0&uploaded=0&peer_id=%2DqB00000000000000001&port=17548&left=0&event=completed&compact=0"), fragment: None }, source: hyper::Error(Connect, ConnectError("tcp connect error", Os { code: 111, kind: ConnectionRefused, message: "Connection refused" })) }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

After implementing the issue https://github.com/torrust/torrust-tracker/issues/680 we could catch the error and continue with the execution of the Tracker Checker, checking other services. CUrrently, the checker is halted without finishing.