Better port permission setup and errors

[zachfeldman]

This PR achieves two things:

• Better error message for when you don't have the right permissions to connect to the LED matrix

Before:

zach@zach-laptopw:~/dev/inputmodule-rs (main)$ ./target/x86_64-unknown-linux-gnu/debug/inputmodule-control led-matrix --clock
Current Time = 22:30
thread 'main' panicked at 'Failed to open port: Error { kind: Io(PermissionDenied), description: "Permission denied" }', inputmodule-control/src/inputmodule.rs:392:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

After:

zach@zach-laptopw:~/dev/inputmodule-rs (main)$ ./target/x86_64-unknown-linux-gnu/debug/inputmodule-control led-matrix --clock
Current Time = 23:46
thread 'main' panicked at 'Permission denied, couldn't access input module port. Ensure that udev rule is installed or the port the module is on is otherwise accessible.', inputmodule-control/src/inputmodule.rs:398:80
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

• A udev rule (per @kiram9 's suggestion) to give the right permissions to connect to the LED matrix by opening up its port and an addition to the README to make it clear to users how to install this rule (eventually we can make this part of an install script)

[zachfeldman]

@JohnAZoidberg just rebased this - could we merge it soon potentially?

[JohnAZoidberg]

Sorry for the long wait @zachfeldman. Thanks for the contribution! :D

[JohnAZoidberg]

I tested it and adjusted a bit according to the best practices recommended by the arch wiki. We don't need to open the permissions to 666. 660 is enough with udev giving access to the logged in user.

[zachfeldman]

Hey no problem @JohnAZoidberg I know it's been a busy time for your team! Thanks so much for taking the time to review this/get it in.