[CAL-3321] better error when API is trying to book event type that has custom questions

[PeerRich]

when calling the booking api without the custom questions, itll throw a generic error

_CAL-3321

[TenzDelek]

@PeerRich hello there, I would like to contribute to that, kinda new to open source and all, from what I understand , are you implying to make a better error message when we call the booking api or something else, can the problem statement be more specific regarding where the code problem is ( is that the one in the v1 folder?).