Get context in `grafserv.maskError`
### Feature description

It'll be usefull to have an ability to get `Grafast.Context` in the `GraphileConfig.GrafservOptions.maskError`. For instance:

```ts
grafserv: {
    ...
    maskError: (err, context) => {
        ...
     }
  },
 ```

### Motivating example

This can be useful for errors logging. For example:

```ts
grafserv: {
    ...
    maskError: (err, { userId }) => {
       if (e.originalError instanceof db.NotExactlyOneError) {
           logger.warn(
             "Attempting to access a prohibited or non-existent resource",
             userId
           );
       }
       return e;
     }
  },
 ```

### Supporting development

I [tick all that apply]:

<!-- To tick a checkbox, change it from `[ ]` to `[x]` -->

- [ ] am interested in building this feature myself
- [ ] am interested in collaborating on building this feature
- [ ] am willing to help testing this feature before it's released
- [ ] am willing to write a test-driven test suite for this feature (before it exists)
- [ ] am a [Graphile sponsor](https://www.graphile.org/sponsor/) ❤️
- [ ] have an active [support or consultancy contract](https://www.graphile.org/support/) with Graphile
