New errors for create_notification, items_by_column_values and the people column

We will be introducing the following errors to the create_notification mutation, items_by_column_values query and the people column.

New error for create_notification mutation:

In one week, sending an invalid user ID in a create_notification mutation will result in a 500 error.

A valid user ID must be an integer.

New error when querying items by column values:

In one week, providing a nonexistent board in a “items_by_column_values” query will result in a 500 error.

Improved error when populating the people column:

In one week, you will see a descriptive error when you try to add a user to a person column when the user is not assigned to the board.

Previously, this would result in a nonspecific 500 error.