Better multi/exec results (suggestion)?

Hello.

As for versions 1.x and 2.x-rc transaction multi/exec results are returned in [Error, [Error1, Value1], ... ] format. With promised based API it looks very confusing, for example:

ioredis.multi().get('key1').get('key2').exec()
  .spread( ([e1, val1], [e2, val2]) => {
    /* NO ERRORS(?) */
  })
  .catch(e => {
    /* ERROR HANDLING */
  });

It is unclear what should e1 and e2 mean in this context. As far as i understand, partially applied transaction should happen only when arguments count is wrong, or command is used on a wrong value type. So such errors should never occur when redis is used correctly. Will it be better to have something like this?

ioredis.multi().get('key1').get('key2').exec()
  .spread( (val1, val2) => {
    /* NO ERRORS */
  })
  .catch(e => {
    /* ERROR HANDLING, e may have a field with transaction results
     * like this [ [Error1, Value1], ... ] */
  });

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[lo1tuma]

I would also like to see a better transaction result format for the promise-based API.

What do you think about introducing a custom error type, e.g. PartiallyAppliedTransactionError which could have additional fields that point exactly to the commands that have been applied or failed, for example:

redis.multi().set('key1', 'val1').hget('key1', 'val1').exec()
    .catch(error => {
        if (error instanceof redis.PartiallyAppliedTransactionError) {
            console.log(error.commandErrors); // ⇨ [null, [ReplyError: WRONGTYPE Operation against a key holding the wrong kind of value]]
            console.log(error.commandResults); // ⇨ ['OK', null]
        }
    });