assemble_fibermap bug for "unpositioned" exposures

[sbailey]

We have a bug in assemble_fibermap counting of positioning iterations for "unpositioned" exposures, e.g.

log: /global/cfs/cdirs/desi/spectro/redux/daily/run/scripts/night/20201220/prestdstar-20201220-00069082-a0123456789-37348434.log

INFO:desi_proc:168:<module>: Creating fibermap /global/cfs/cdirs/desi/spectro/redux/daily/preproc/20201220/00069082/fibermap-00069082.fits
INFO:util.py:73:runcmd: Tue Dec 22 10:41:55 2020
INFO:util.py:74:runcmd: RUNNING: assemble_fibermap -n 20201220 -e 69082 -o /global/cfs/cdirs/desi/spectro/redux/daily/preproc/20201220/00069082/fibermap-00069082.fits
INFO:fibermap.py:405:assemble_fibermap: Night 20201220 spectro expid 69082
INFO:fibermap.py:406:assemble_fibermap: Raw data file /global/cfs/cdirs/desi/spectro/data/20201220/00069082/desi-00069082.fits.fz
INFO:fibermap.py:407:assemble_fibermap: Fiberassign file /global/cfs/cdirs/desi/spectro/data/20201220/00069082/fiberassign-080618.fits
INFO:fibermap.py:408:assemble_fibermap: Platemaker coordinates file /global/cfs/cdirs/desi/spectro/data/20201220/00069082/coordinates-00069082.fits
INFO:fibermap.py:409:assemble_fibermap: Guider file /global/cfs/cdirs/desi/spectro/data/20201220/00069082/guide-00069082.fits.fz
INFO:fibermap.py:423:assemble_fibermap: 5000/5000 fibers in coordinates file
WARNING:fibermap.py:442:assemble_fibermap: No FIBER_X/Y or DELTA_X/Y information from platemaker
Traceback (most recent call last):
 File "/global/common/software/desi/cori/desiconda/20200801-1.4.0-spec/code/desispec/master/bin/assemble_fibermap", line 22, in <module>
  fibermap = assemble_fibermap(args.night, args.expid, force=args.force)
 File "/global/common/software/desi/cori/desiconda/20200801-1.4.0-spec/code/desispec/master/py/desispec/io/fibermap.py", line 449, in assemble_fibermap
  expflag = pm[f'FLAGS_EXP_{numiter-1}']
 File "/global/common/software/desi/cori/desiconda/20200801-1.4.0-spec/conda/lib/python3.8/site-packages/astropy/table/table.py", line 1602, in __getitem__
  return self.columns[item]
 File "/global/common/software/desi/cori/desiconda/20200801-1.4.0-spec/conda/lib/python3.8/site-packages/astropy/table/table.py", line 239, in __getitem__
  return OrderedDict.__getitem__(self, item)
KeyError: 'FLAGS_EXP_-1'
 Outputs
  /global/cfs/cdirs/desi/spectro/redux/daily/preproc/20201220/00069082/fibermap-00069082.fits
INFO:util.py:97:runcmd: Tue Dec 22 10:42:00 2020
CRITICAL:util.py:99:runcmd: FAILED assemble_fibermap -n 20201220 -e 69082 -o /global/cfs/cdirs/desi/spectro/redux/daily/preproc/20201220/00069082/fibermap-00069082.fits