Improve error messages on missing files

[SebastienGllmt]

If you have file missings, the errors are really generic and don't help much figuring out which files was missing. This seems like an open issue with Rust itself. Until they improve the missing file error message, we can add our own error message.

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Io(Os { code: 2, kind: NotFound, message: "No such file or directory" })', tasks/src/execution_plan.rs:7:49

[SebastienGllmt]

I did some of these at some point. Not sure if there are any left