Perform CORS handling when responding to Symfony HTTP Exceptions

## Perform CORS handling when responding to Symfony HTTP Exceptions

Currently, the responses given in [Synapse\Application\Routes](https://github.com/synapsestudios/synapse-base/blob/master/src/Synapse/Application/Routes.php) aren't caught by the CORS middleware. This results in inappropriate browser console errors.

(e.g. 404? CORS error. 401 Unauthorized? CORS error.)

### Acceptance Criteria
1. Responses given in #133 in response to various Symfony Http Exceptions return appropriate CORS header. (`Access-Control-Allow-Origin`, `Access-Control-Allow-Credentials`, possibly others)

### Tasks
- Determine if CORS library can handle it.
- If not, manually add header(s).

### Additional Notes
- [CORS Library](https://github.com/jdesrosiers/silex-cors-provider/blob/d157466a40bd615f8cbc7158e930118f7e69a772/src/JDesrosiers/Silex/Provider/CorsServiceProvider.php)