error handling

[Rudxain]

• [ ] Handle possible std{in, out, err} stream errors
• [x] Avoid panic if file doesn't exist, and print error message to stderr
• [x] Avoid panic if path is a directory, and eprint
• [x] Avoid panic if there's no permission to read a file, and eprint
• [x] Avoid panic if --length <n> cannot be parsed to usize (handled by clap)
• [x] Return meaningful exit codes

[Rudxain]

I definitely need to read this lmao