Schema validation failed: Data path "" should NOT have additional properties

### Bug Report or Feature Request (mark with an `x`)
```
- [x] bug report -> please search issues before submitting
- [ ] feature request
```

### Area
```
- [ ] devkit
- [x] schematics
```

### Versions
```
Angular CLI: 6.0.8
Node: 8.11.2
OS: win32 x64
Angular: 6.0.7
... animations, common, compiler, compiler-cli, core, forms
... http, language-service, platform-browser
... platform-browser-dynamic, router

Package Version
-----------------------------------------------------------
@angular-devkit/architect 0.6.8
@angular-devkit/build-angular 0.6.8
@angular-devkit/build-optimizer 0.6.8
@angular-devkit/core 0.6.8
@angular-devkit/schematics 0.6.8
@angular/cli 6.0.8
@ngtools/webpack 6.0.8
@schematics/angular 0.6.8
@schematics/update 0.6.8
rxjs 6.2.1
typescript 2.7.2
webpack 4.8.3
```

### Repro steps

1. Add `projects/architect//options/invalid` options to `options` in `angular.json` eg
```
"build": {
"builder": "@angular-devkit/build-angular:browser",
"options": {
"invalid": "value",
...
}
}
```
2. Run `ng build`

### Actual functionality

```
Schema validation failed with the following errors:
Data path "" should NOT have additional properties(invalid).
```

### Desired functionality

```
Schema validation failed with the following errors:
Data path "path/to/invalid/option" should NOT have additional properties(invalid).
```

Beginne damit, die Schema-Validierung während `ng build` für die in der Reproduktion gezeigten Build-Optionen in `angular.json` nachzuverfolgen. Bestätige, wo die ungültige Option gemeldet wird, und aktualisiere die Diagnose so, dass sie den relevanten Optionspfad enthält. Überprüfe anschließend, dass die Ausgabe der gewünschten Funktionalität entspricht.

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