Fix description deprecation warnings

The description and cause methods in Error were soft deprecated in Rust 1.27 and hard deprecated in Rust 1.42 [1]. This change removes the use of these deprecated methods and fixes three deprecation warnings.

You can verify that the test output has not changed with the following steps:

1. Create the file /etc/pkg.d/localtest
2. Add the line localhost:1234 to /etc/pkg.d/localtest
3. Run Netcat on port 1234 in a new terminal to simulate a server:

   $ while true; do { echo -e 'HTTP/0 200 OK\r\n'; } | nc -l 1234; done

   The server returns an invalid HTTP version on purpose.
4. Try to fetch a package, e.g. pixelcannon:

   $ cargo run -- fetch pixelcannon

5. Notice that you get the following error message with or without the changes:

   * Requesting http://localhost:1234/x86_64-unknown-linux-gnu/pixelcannon.sig
   pkg: fetch: pixelcannon: failed: Invalid HTTP version specified