JSON error handling

This PR starts the consolidation process of error handling throughout the backend. Now, errors are raised as exceptions that propagate to route-wide or app-wide handlers attached via register_error_handler.

Example JSON:

{
  "object": "error",
  "type": "invalid_request_error",
  "error_code": "validation_error",
  "status_code": 400,
  "friendly_message": "Some message we can display to the user here",
  "params": "something can go here, an array, object, etc"
}