[Example code] Improve error message when panicking due to not being able to find certificates

[paulcdejean]

Before:

thread 'main' panicked at src/args.rs:58:10:
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

After:

thread 'main' panicked at src/args.rs:60:13:
Example certificates not found in /Users/paul/gitseed/gitseed/listener/examples/cert.crt
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace