Check assets folder on start

[Vinatorul]

I think it will be nice to check assets or just assets folder on start to exit gracefully with hint run make assets before cargo run instead of:

thread '<main>' panicked at 'Can`t open file 'assets/DroidSerif-Regular.ttf' (No such file or directory (os error 2))', src/visualizer/src/fs.rs:14
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: Process didn't exit successfully: `target/debug/zoc` (exit code: 101)

[ozkriff]

@Vinatorul Is it ok now? :)

[Vinatorul]

Great, thanks!