Mongod Deletion: Cleanup Observation Error

[problame]

There's no FOREIGN KEY REF mongods(observation_error) ON DELETE CASCASE in the msp_errors table, hence it won't be automatically deleted.

The clean way of doing this would be having such foreign keys, however, #14 yields the same issue for msp_errors related to slaves, hence, we would need a reference into the slaves table, too.