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

The payload does not name a file, test, or entry point. Locate the project-item query that currently surfaces GraphqlResponseError, inspect its missing-item response, and verify that the completed behavior raises GitHubProjectItemNotFoundError specifically for that case.

Written by the indexing model from the issue text.