Introduce fs_err as a replacement for fs

[GabrielSimonetto]

Resolves #56

I tried reproducing some errors and see if I can see the difference between before and after, but most errors just look the

• - Erase a file that was already checked to exist

// On command.rs, and removing the .zip while it sleeps:
        Command::Decompress { files, output_folder } => {
            std::thread::sleep(std::time::Duration::from_secs(10));

$ cargo run d test_dir.zip
thread 'main' panicked at 'failed to open file `/home/gabriel/programas/ouch/test_dir.zip`', src/error.rs:177:45
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

• - Change the permission of something

// This one doesn't require sleep
$ chmod 333 test_dir.zip

$ cargo run d test_dir.zip
thread 'main' panicked at 'not yet implemented', src/error.rs:159:40
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

• - Delete something while writing

I was not able to reproduce this one