Add redis error codes to the errors

[BridgeAR]

This adds redis error codes to the returned errors. We might as well add error classes like parser errors, network errors and similar (the later do not have any error codes yet).

Fixes partly #695 and closes #538

[skeggse]

Possible test for error codes:

it('converts lua error to an error response', function (done) {
  helper.serverVersionAtLeast.call(this, client, [2, 6, 0]);
  client.evalsha("ffffffffffffffffffffffffffffffffffffffff", 0, function(err) {
    assert(err.code === "NOSCRIPT");
    helper.isError()(err);
    done();
  });
});

[BridgeAR]

Thx @skeggse

[skeggse]

No problem