BUG: Weird error while downloading GRB skymaps

[virajkaram]

Describe the bug
I try running snipergw for a GRB230430A and it throws an uninformative error when it attempts to download the map - it is not clear whether no map is available or there is a bug in the code.

To Reproduce
Steps to reproduce the behavior:

1. Run python -m snipergw -e GRB230430A
2. See error -

INFO:snipergw.skymap:Found multiple events. Will choose the one corresponding the GRB letter A
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn230430196/current/ ['?C=N;O=D', '?C=M;O=A', '?C=S;O=A', '?C=D;O=A', '/FTP/fermi/data/gbm/triggers/2023/bn230430196/', 'glg_cspec_b0_bn230430196_v00.pha', 'glg_cspec_b1_bn230430196_v00.pha', 'glg_cspec_n0_bn230430196_v00.pha', 'glg_cspec_n1_bn230430196_v00.pha', 'glg_cspec_n2_bn230430196_v00.pha', 'glg_cspec_n3_bn230430196_v00.pha', 'glg_cspec_n4_bn230430196_v00.pha', 'glg_cspec_n5_bn230430196_v00.pha', 'glg_cspec_n6_bn230430196_v00.pha', 'glg_cspec_n7_bn230430196_v00.pha', 'glg_cspec_n8_bn230430196_v00.pha', 'glg_cspec_n9_bn230430196_v00.pha', 'glg_cspec_na_bn230430196_v00.pha', 'glg_cspec_nb_bn230430196_v00.pha', 'glg_ctime_b0_bn230430196_v00.pha', 'glg_ctime_b1_bn230430196_v00.pha', 'glg_ctime_n0_bn230430196_v00.pha', 'glg_ctime_n1_bn230430196_v00.pha', 'glg_ctime_n2_bn230430196_v00.pha', 'glg_ctime_n3_bn230430196_v00.pha', 'glg_ctime_n4_bn230430196_v00.pha', 'glg_ctime_n5_bn230430196_v00.pha', 'glg_ctime_n6_bn230430196_v00.pha', 'glg_ctime_n7_bn230430196_v00.pha', 'glg_ctime_n8_bn230430196_v00.pha', 'glg_ctime_n9_bn230430196_v00.pha', 'glg_ctime_na_bn230430196_v00.pha', 'glg_ctime_nb_bn230430196_v00.pha', 'glg_lc_all_bn230430196_v01.gif', 'glg_lc_chan12_bn230430196_v01.pdf', 'glg_lc_chan34_bn230430196_v01.pdf', 'glg_lc_chan567_bn230430196_v01.pdf', 'glg_lc_chantot_bn230430196_v01.pdf', 'glg_lc_hires12_bn230430196_v01.gif', 'glg_lc_hires34_bn230430196_v01.gif', 'glg_lc_hires567_bn230430196_v01.gif', 'glg_lc_lores12_bn230430196_v01.gif', 'glg_lc_lores34_bn230430196_v01.gif', 'glg_lc_lores567_bn230430196_v01.gif', 'glg_lc_medres12_bn230430196_v01.gif', 'glg_lc_medres34_bn230430196_v01.gif', 'glg_lc_medres567_bn230430196_v01.gif', 'glg_lc_tot_bn230430196_v01.pdf', 'glg_lc_zxradec_bn230430196_v01.gif', 'glg_tcat_all_bn230430196_v00.fit', 'glg_trigdat_all_bn230430196_v02.fit', 'glg_tte_b0_bn230430196_v00.fit', 'glg_tte_b1_bn230430196_v00.fit', 'glg_tte_n0_bn230430196_v00.fit', 'glg_tte_n1_bn230430196_v00.fit', 'glg_tte_n2_bn230430196_v00.fit', 'glg_tte_n3_bn230430196_v00.fit', 'glg_tte_n4_bn230430196_v00.fit', 'glg_tte_n5_bn230430196_v00.fit', 'glg_tte_n6_bn230430196_v00.fit', 'glg_tte_n7_bn230430196_v00.fit', 'glg_tte_n8_bn230430196_v00.fit', 'glg_tte_n9_bn230430196_v00.fit', 'glg_tte_na_bn230430196_v00.fit', 'glg_tte_nb_bn230430196_v00.fit']
INFO:snipergw.skymap:Downloading skymap and saving to /Users/viraj/Data/snipergw/skymaps/glg_tte_nb_bn230430196_v00.fit
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/Users/viraj/snipergw/snipergw/__main__.py", line 48, in <module>
    run_snipergw(
  File "/Users/viraj/snipergw/snipergw/run.py", line 36, in run_snipergw
    skymap = Skymap(event_config=event)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/viraj/snipergw/snipergw/skymap.py", line 57, in __init__
    self.skymap_path, self.event_name = self.get_grb_skymap(
                                        ^^^^^^^^^^^^^^^^^^^^
  File "/Users/viraj/snipergw/snipergw/skymap.py", line 156, in get_grb_skymap
    wget.download(final_link, str(skymap_path))
                  ^^^^^^^^^^
UnboundLocalError: cannot access local variable 'final_link' where it is not associated with a value

Expected behavior
Downloads map, or raises a reasonable exception if not available.

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Additional context
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