MAINT: refactor ufunc error object handling

_get_global_ext_obj: retrieves global ufunc object
_get_bufsize_errmask: get only bufsize and errormask from ufunc object
_extract_pyvals: handle NULL extobj

PyUFunc_GetPyValues implemented as _get_global_ext_obj +_extract_pyvals
drop unused first_error variable.

fix errobj memory leak in previous commit.

add some test for the extobj and warning path, the warning tests are
disabled like the raising path as they fail on a bunch of platforms.