Opaque error when data sources is missing a column #46
Description
When the "data sources" tab is missing a column (e.g., column G on old versions of the sheet), it exceptions with an opaque error:
>>> doc = es.excel_to_sbol(wb)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:\Python39\site-packages\sbol_utilities\excel_to_sbol.py", line 426, in excel_to_sbol
basic_parts, composite_parts, linear_products, final_products, source_table = read_metadata(wb, doc, config)
File "C:\Python39\site-packages\sbol_utilities\excel_to_sbol.py", line 110, in read_metadata
source_table = {row[config['source_name_col']].value: row[config['source_uri_col']].value
File "C:\Python39\site-packages\sbol_utilities\excel_to_sbol.py", line 112, in <dictcomp>
if row[config['source_literal_col']].value}
IndexError: tuple index out of range
It should at least be more graceful about its crash.