Improve database value decoding diagnostics

[groue]

This PR makes as much information as possible available when a database value conversion error happens:

struct Player: Decodable, FetchableRecord {
    var id: Int64
    var name: String
    var score: Int
}

// fatal error: could not convert database value NULL to String
// (column: `name`, column index: 1, row: [id:42 name:NULL score:100],
// sql: `SELECT * FROM player WHERE id = ?`, arguments: [42])
let player = try Player.fetchOne(db, key: 42)

In the example above, the error message helps understanding that the Player.name property should be made optional, or that the player.name database column should be changed so that it never contains null values.

And in the following example, it is easy to spot and fix the typo:

struct Player: Decodable, FetchableRecord {
    var id: Int64
    var name: String
    var scor: Int
}

// fatal error: could not read Int from missing column `scor`
// (row: [id:42 name:"Arthur" score:100],
// sql: `SELECT * FROM player WHERE id = ?`, arguments: [42])
let player = try Player.fetchOne(db, key: 42)

With previous GRDB versions, it was sometimes difficult to know which columns or properties would need fixing.