Show better error message when creating `ForeignKey::PrimaryKey` with non-existent ID

[ElijahAhianyo]

Currently the example below:

use cot::db::ForeignKey;
use cot::{db, Auto, LimitedString};

#[model]
pub struct Product {
    #[model(primary_key)]
    id: Auto<i64>,
    #[model(unique)]
    sku: LimitedString<64>,
    name: LimitedString<255>,
    price_cents: i64,
    stock: i32,
    is_available: bool,
}

#[model]
pub struct Order {
    #[model(primary_key)]
    id: Auto<i64>,
    customer: ForeignKey<Customer>,
    product: ForeignKey<Product>,
    quantity: i32,
    is_fulfilled: bool,
}


async fn save_order(db: Database) -> cot::Result<()> {
    let mut order = Order {
        id: Auto::default(),
        product: ForeignKey::PrimaryKey(Auto::fixed(1)),
        quantity: 1,
        is_fulfilled: false,
    };
    order.save(db).await?;
}
...

Will return a UniqueViolation Error when the product_id being referenced here does not exist. It should say the ID does not exist.