cmdline_parse.py: check if file exist

If file passed as input to tesouro_direto.py does not exist, it generates a rather cryptic error:

[leonardo@leo spinparse]$ ./spinparse/parser/tesouro_direto.py -i 2019-12.xls
Traceback (most recent call last):
  File "./spinparse/parser/tesouro_direto.py", line 92, in <module>
    td.to_csv(args.input, args.outpath)
  File "./spinparse/parser/tesouro_direto.py", line 74, in to_csv
    self._preprocess(fname, full_out_fpath, "csv")
  File "./spinparse/parser/tesouro_direto.py", line 68, in _preprocess
    self._parse_in_contents()
  File "./spinparse/parser/tesouro_direto.py", line 52, in _parse_in_contents
    dfs = pd.read_html(self._full_in_fname, skiprows=1)
  File "/usr/lib/python3.8/site-packages/pandas/io/html.py", line 1090, in read_html
    return _parse(
  File "/usr/lib/python3.8/site-packages/pandas/io/html.py", line 912, in _parse
    raise_with_traceback(retained)
  File "/usr/lib/python3.8/site-packages/pandas/compat/__init__.py", line 47, in raise_with_traceback
    raise exc.with_traceback(traceback)
ValueError: No tables found

Start at spinparse/parser/tesouro_direto.py, following the -i input from the command-line entry point into to_csv and _preprocess. Reproduce the shown command with a missing file and make the resulting error identify the missing input instead of reaching pandas.read_html; confirm the behavior with the same invocation.