Throw `GitHubProjectItemNotFoundError` when project item could not be found in query

### Please avoid duplicates

- [x] I checked [all open feature requests](https://github.com/gr2m/github-project/issues?q=is%3Aissue+is%3Aopen+label%3Afeature) and none of them matched my problem.

### What’s missing?

A custom error for when an item is not found

### Why?

currently a GraphqlResponseError is thrown, with an error looking like this

```js
{
type: 'VALIDATION',
path: [ 'BU' ],
locations: [ { line: 4, column: 11 } ],
message: 'The item does not exist in the project'
}
```

### Alternatives you tried

The error can be caught and identified by manually looking through all errors included in the response

### Would you be interested in contributing the feature?

- [ ] yes

Le payload ne nomme ni fichier, ni test, ni point d’entrée. Localisez la requête d’élément de projet qui fait actuellement apparaître GraphqlResponseError, examinez sa réponse lorsqu’un élément est manquant et vérifiez que le comportement final lève spécifiquement GitHubProjectItemNotFoundError dans ce cas.

Rédigé par le modèle d'indexation à partir du texte de l'issue.