NodeJS sqlite3 get error message

How to get error message from "err" variable? When I dump that variable it contains

{ [Error: SQLITE_CONSTRAINT: UNIQUE constraint failed: _test.Id] errno: 19, code: 'SQLITE_CONSTRAINT' }

I need to get "Error: SQLITE_CONSTRAINT: UNIQUE constraint failed: _test.Id" from "err" variable. I'm executing this query.

db.run("INSERT INTO _test (Var1) VALUES (?)", '1', function (err) {
        if (err) {
            console.log(err);
            return;
        }
});

Alredy tried err[0], print out all properties with "for", but it returns only "errorno" and "code" properties.