Migration endpoint fails silently when getting an unexisting migration id

to reproduce, run a nuxeo instance and do the following:
```curl -iu Administrator:Administrator \
http://localhost:8080/nuxeo/api/v1/management/migration/doesNotExistMigration```
You will get a 204 (no content) response.

The Migration endpoint should return a 404 (not found) status code like the ProbesObject does for example.