When settings.json is missing, print a reasonable error message

[njvack]

Right now it does something like

Traceback (most recent call last):
  File "/study/bam/apps/conda3.9/bin/masterfile", line 8, in <module>
    sys.exit(main())
  File "/study/bam/apps/conda3.9/lib/python3.9/site-packages/masterfile/scripts/masterfile.py", line 101, in main
    return extract.main(args)
  File "/study/bam/apps/conda3.9/lib/python3.9/site-packages/masterfile/scripts/extract.py", line 54, in main
    formatted = format_dataframe_for_masterfile(
  File "/study/bam/apps/conda3.9/lib/python3.9/site-packages/masterfile/scripts/extract.py", line 82, in format_dataframe_for_masterfile
    col_rx = col_match_regex(mf, input_index_col)
  File "/study/bam/apps/conda3.9/lib/python3.9/site-packages/masterfile/scripts/extract.py", line 105, in col_match_regex
    re.escape(index_col), component_col_regex(len(mf.components)))
TypeError: object of type 'NoneType' has no len()

which is not helpful