Angular integration emits null instead of actual error from Watchdog

[kubaklatt]

The (error) output in ckeditor-angular sometimes emits null, which could be confusing. The error is actually assigned to itemError in the Watchdog code:

• https://github.com/ckeditor/ckeditor5/blob/ck/12952/packages/ckeditor5-watchdog/src/contextwatchdog.ts#L282

But it is ignored in the Angular integration. This could be improved so that the actual error is properly emitted:

• https://github.com/ckeditor/ckeditor5-angular/blob/master/src/ckeditor/ckeditor.component.ts#L442