API cleanup

[marcua]

• Return JSON on errors
• Return 201 on create, 404 on not found (ended up skipping 404 on on not found for now)
• already appears to have been fine, though perhaps I forgot what the problem was More informative errors for CRUD operations
  • RowNotFound for get_XYZ
  • Already exists for create_XYZ
• IntantiatedEntity and InstantiatedDatabase return integers for entity_type and db_type --- should they be serialized as more human-readable strings?

[marcua]

Resolved by #57