BUG: fix crash on default errobj

Missing check for optional NULL argument, the case can only happen if
the error mask is 0 (the old default).
closes gh-3962
ENH: merge UFUNC_ERR_DEFAULT2 variable into UFUNC_ERR_DEFAULT

It seems it was added as a precaution to avoid breaking the rather
complicated ufunc code. It effectively disabled the skipping of the
dictionary lookup for the default values for the common case and leading
to hardly tested code in third party libraries like pandas which set the
error state to numpys old default (ignore all).
The skipping improves scalar performance by 2.5-5%.
Merge pull request #3963 from juliantaylor/default-errobj-crash

Default errobj crash