added error handling for GCPadNew.ini not existing

[ZXSwire3]

Solves the issue if the user does not have the GCPadNew.ini file in their Slippi config.

I was having this issue and it was not obvious how to fix it till I looked into the code to find the real issue was that I didn't have a the GCPadNew.ini file in my Slippi config. Here is the error that is given without the change made.

thread 'main' panicked at /Users/benfoltz/.cargo/registry/src/index.crates.io-6f17d22bba15001f/macboxx-0.1.5/src/config.rs:13:79:
called `Result::unwrap()` on an `Err` value: Io(Os { code: 2, kind: NotFound, message: "No such file or directory" })
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I'm not sure if this file is created with Slippi by default but this will prevent that issue from happening to more users.