MANT: more informative errors for 'axis' argument of ufuncs

This follows from the discussion in #5196, and unifies the errors returned by `PyArray_ConvertMultiAxis` in `conversion_utils.c`, with the similar code in `ufunc_object.c` to handle the `axis` parameter to ufunc reduction methods.
> integer overflows

Is this a serious concern? the largest possible axis index is 31... (not `2**31`, just 31).

> A more elaborate possibility is to add PyArray_ConvertMultiAxis to the API, and adapt it slightly (return naxes also) so that it can be used here directly.

I'll point out that this is Yet Another case where we are considering exposing boring utility functions with funky calling conventions in the API solely so we can use them internally in both multiarray and umath.