MAINT: Give a more helpful error for bad axis specifications.

This specifically addresses passing things like axis=[0,1] which
gave an error message that an int was required.  The real error
was that if the axis isn't None or a tuple, it was simply wraped
in a tuple, so the ufunc that is eventually called ended up with
a list.  The error message is matches that in
core/src/multiarray/conversion_utils.c::PyArray_ConvertMultiAxis.