Catch AccessDeniedHttpExceptions

## Catch AccessDeniedHttpExceptions

### Acceptance Criteria
1. When a user visits an endpoint but is restricted from access via access rules, the AccessDeniedHttpException is caught before it gets to Silex.
1. The Exception is caught and simply turned into a 403 JSON response with the following body:

```
{"error":"Access denied"}
```

### Tasks
- Catch AccessDeniedHttpExceptions in `Synapse\Application\Routes` and return the JSON response.