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:
    ```console
    $ 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`:
   ```console
   $ cargo run -- fetch pixelcannon
   ```
5. Notice that you get the following error message with or without the changes:
   ```console
   * Requesting http://localhost:1234/x86_64-unknown-linux-gnu/pixelcannon.sig
   pkg: fetch: pixelcannon: failed: Invalid HTTP version specified
   ```

[1]: https://blog.rust-lang.org/2020/03/12/Rust-1.42.html#errordescription-is-deprecated