Improve error message if config file not found.

```
$ kmip2pkcs11 --stderr --config /etc/kmip2pkcs11/config.tom

thread 'main' panicked at src/config.rs:105:62:
called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Aborted
```