Describe the bug

Passing an invalid filename to hx causes it to crash on a Result:

To Reproduce
Steps to reproduce the behavior:

$ hx toto
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }', $HOME/.config/rust/cargo/registry/src/github.com-1ecc6299db9ec823/hx-0.3.0/src/lib.rs:238:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Expected behavior

I expected the program to tell me I made a mistake.

Desktop (please complete the following information):

• OS: macOS 10.15.7

Additional context

$ hx --version
hx 0.3.0

I'll probably have the time to open a PR for a fix tomorrow if wanted.