Catchlist_todo

We have a bunch of exceptions that make use of a superfluous "message" attribute:

class TaskValidationError(ValidationError):
    def __init__(self, message: str):
        self.message = message
        super().__init__(message)

This is unnecessary. Exceptions store their message in arg[0] and you can just do str(e) to fetch them.
You can just do this:

            raise ValidationError(f"Remote calendar {remote_uid} not found.")

And then later:

        return jsonify({"error": str(e)}), 401

We should replace all instances of e.message with str(e), and then remove the class definition for exceptions entirely (unless some of them have something other than saving a "message" attribute.