Exception in dev env when only prod is set

Based on documented config:
```yml
prod:
  amg_sentry:
    enabled: true
    dsn: 'http://public:secret@sentry.example.com:9000/[PROJECT_ID]'
    logger: 'custom-logger-name'
```
When:
```php
amgSentry::sendMessage('Message title');
```
was called in dev env exception is throw:
```php
throw new Exception('Please configure amgSentryPlugin in your app.yml (use model in "amgSentryPlugin/config/app.yml")');
```

Sorry for my bad English