no WISE_VAR_QSO secondary targets in SV2

[moustakas]

This line and this line in qso_cat_utils will crash because there were no WISE_VAR_QSO secondary targets in SV2.

from desitarget.sv2.sv2_targetmask import scnd_mask as sv2_scnd_mask
sv2_scnd_mask.mask('WISE_VAR_QSO')
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
Cell In[2], line 1
----> 1 sv2_scnd_mask.mask('WISE_VAR_QSO')

File /global/common/software/desi/perlmutter/desiconda/20240425-2.2.0/code/desiutil/main/py/desiutil/bitmask.py:215, in BitMask.mask(self, name_or_num)
    213 mask = 0
    214 for name in name_or_num.split('|'):
--> 215     mask |= self._bits[name].mask
    216 return mask

KeyError: 'WISE_VAR_QSO'

It's a low-priority fix, but I tripped on it because I copied the logic used here into FastSpecFit in desihub/fastspecfit#201.

FWIW, the fix I propose is something like:

if 'WISE_VAR_QSO' in sv2_scnd_mask.names():
    # do stuff