Match DatabaseError on ResultCode

[groue]

This pull request makes it possible to match database errors on their result code:

do {
    try Player(...).insert(db)
} catch ResultCode.SQLITE_CONSTRAINT_FOREIGNKEY {
    // foreign key constraint error
} catch ResultCode.SQLITE_CONSTRAINT {
    // any other constraint error
} catch {
    // any other database error
}

Edit: further GRDB 5 commits expose the result codes on DatabaseError itself:

do {
    try Player(...).insert(db)
} catch DatabaseError.SQLITE_CONSTRAINT_FOREIGNKEY {
    // foreign key constraint error
} catch DatabaseError.SQLITE_CONSTRAINT {
    // any other constraint error
} catch {
    // any other database error
}