Improve error message for invalid executors

[brumhard]

This is just a minor thing so I thought I might as well open a PR and see what you think.

Problem

Currently, when you use languages for code blocks that are either not handled (e.g. I tried shell instead of sh in the beginning) or for which the executor program (like node for js) is missing you receive an error like:

> mask foo
ERROR: No such file or directory (os error 2)

For me this was kinda misleading when starting to use the tool.

Describe the solution

That's why I added some error mapping which translates it to:

> cargo run -- foo
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
     Running `target/debug/mask foo`
ERROR: program 'node' for executor 'js' not in PATH

Testing code

The definition that I used for testing is:

## foo

```js
console.log("bar")
```

Other example

For other programs the new error message might look like:

> cargo run -- lel
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
     Running `target/debug/mask lel`
ERROR: program 'shell' for executor 'shell' not in PATH

which doesn't look great but I still think it's better than the current error.

[jacobdeichert]

This is a nice improvement, thanks for adding a test!

I'll merge it now but probably won't make a new release for at least a week.