Essentially, failure experience is regarded very positively by MBB: if you failed, you took risks, i.e. you have proven your entrepreneurial nature. So, do not be afraid to talk honestly about the times you failed.
However, an important thing to focus on in your answer to this question is what you have learnt from your failure. And I would try to turn these “lessons learnt” into something valuable for consulting. For example, you tried doing everything by yourself and after this failure you really started understanding the importance of team work and really improved on this front. Or, you tried doing everything perfectly and after that failure you understood that it’s important to have trade-offs and to compromise to achieve the results you want (80/20 or “better done than perfect“).
So, be honest, talk about your failures, but remember to highlight some lessons learnt to give your story an overall positive twist.