Assertion won't be formatted correctly in _arguments

[tovacinni]

Sorry for the issue spamming- but here's a small bug report that this assertion needs to be replaced by an Exception to properly display which fields have issues in the defaults. I would submit a PR but I'll probably get the type of Exception to use wrong :D

https://github.com/brentyi/tyro/blob/e88e690e14ffe87e13419fa2b6427bc77c4ae336/tyro/_arguments.py#L198-L201