Crashing in TWOZONE when BACKCORR=OMIT

[lmiller20]

When the XTRACTALG = TWOZONE and BACKCORR = OMIT, CalCOS will crash with the following error:

---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
<ipython-input-10-05b2f5b03a8b> in <module>()
----> 1 calcos.calcos('ldwd01hfq_rawtag_a.fits')~/anaconda/envs/astroconda_3/lib/python3.6/site-packages/calcos/calcos.py in calcos(asntable, outdir, verbosity, find_target, create_csum_image, raw_csum_coords, only_csum, binx, biny, compress_csum, compression_parameters, shift_file, save_temp_files, stimfile, livetimefile, burstfile)
    432 
    433     wav_status = cal.allWavecals()
--> 434     sci_status = cal.allScience()
    435     if sci_status:                      # bad value for aperture keyword
    436         return sci_status~/anaconda/envs/astroconda_3/lib/python3.6/site-packages/calcos/calcos.py in allScience(self)
   2852                                      obs.info)
   2853                     if obs.switches["x1dcorr"] == "PERFORM":
-> 2854                         self.extractSpectrum(obs.filenames)
   2855                         any_x1dcorr = "PERFORM"
   2856                         any_spectroscopic = "PERFORM"~/anaconda/envs/astroconda_3/lib/python3.6/site-packages/calcos/calcos.py in extractSpectrum(self, filenames)
   2920         find_target = self.assoc.cl_args["find_target"]
   2921         extract.extract1D(input, incounts, output,
-> 2922                           find_target=find_target)
   2923 
   2924         # Copy keywords from input (the flt file) to corrtag.~/anaconda/envs/astroconda_3/lib/python3.6/site-packages/calcos/extract.py in extract1D(input, incounts, output, update_input, location, extrsize, find_target)
    205         doExtract(ifd_e, ifd_c, ofd, nelem,
    206                   segments, info, switches, reffiles, is_wavecal,
--> 207                   location, extrsize, local_find_targ)
    208         if switches["fluxcorr"] == "PERFORM":
    209             # Convert net count rate to flux.~/anaconda/envs/astroconda_3/lib/python3.6/site-packages/calcos/extract.py in doExtract(ifd_e, ifd_c, ofd, nelem, segments, info, switches, reffiles, is_wavecal, location, extrsize, local_find_targ)
    645                                             info, wavelength, is_wavecal,
    646                                             user_xdisp_locn, user_xdisp_size,
--> 647                                             local_find_targ)
    648             else:
    649                 cosutil.printMsg("Unknown extraction method, defaulting to", \~/anaconda/envs/astroconda_3/lib/python3.6/site-packages/calcos/extract.py in extractSegmentTwozone(e_data, c_data, e_dq_data, ofd_header, segment, x_offset, sdqflags, sdqouter, snr_ff, exptime, backcorr, axis, hdr, xtract_info, shift2, proftab_info, info, wavelength, is_wavecal, user_xdisp_locn, user_xdisp_size, find_target)
   1551         term1_i = 0.0
   1552     term2_i = np.zeros(ncols, dtype=np.float32)
-> 1553     goodcolumns = np.where(nrows_c_bkg_i > 0)
   1554     term2_i[goodcolumns] = (flat_correction[goodcolumns])**2 * exptime \
   1555         * (total_ccounts[goodcolumns] + \UnboundLocalError: local variable 'nrows_c_bkg_i' referenced before assignment

Rather than having an unhelpful error, we should have CalCOS exit instead with a reason why it is exiting (crash gracefully).