Improve handling of binaries deleted in filesystem

Binaries in Gentics Mesh should not be deleted from the filesystem. However, in the cases where it does happen (no docker volume mounted, backup imported without binaries), Gentics Mesh should provide better feedback and it should be able to recover.

# Proposal

1. Requests for downloading binaries should not return a simple 404 when the file could not be found on the filesystem. Instead, a 500 with a useful message should be returned (Mentioning docker and the backup process as suggestions)
2. When uploading the same binary as the deleted one (same hash), the de-duplication process should additionally check if the binary really exists in the filesystem. If not, the uploaded binary should be saved.
3. The [consistency checks](https://getmesh.io/docs/api/#admin_consistency_check_get) should check for missing files in the filesystem.