Refactor PersistenceError.recordNotFound

[groue]

In GRDB 3 PersistenceError was defined as below:

// GRDB 3
public enum PersistenceError: Error {
    /// Thrown by MutablePersistableRecord.update() when no matching row could be
    /// found in the database.
    case recordNotFound(MutablePersistableRecord)
}

With such a definition, one can only build PersistenceError.recordNotFound with a fully defined record instance. It is impossible to throw this error with partial information. But some apps need that.

This PR changes the definition of PersistenceError to:

// GRDB 4
public enum PersistenceError: Error {
    case recordNotFound(databaseTableName: String, key: [String: DatabaseValue])
}

When you catch this error, you get the table and the primary key of the unfound record:

do {
    let place = Place(id: nil, name: "Valencia")
    try place.update(db)
} catch {
    print(error)
    // prints "Key not found in table place: [id:NULL]"
}