UnicodeDecodeError when trying to convert non-utf-8 file

[smathot]

This error came in as an automated bug report. It appears to happen when trying to convert a non-utf8 encoded file. This should be dealt with more gracefully.

Traceback (most recent call last):
  File "/Applications/OpenSesame.app/Contents/Resources/share/opensesame_extensions/oswebext/oswebext_widget.py", line 174, in _convert_results
    dm = data.parse_jatos_results(jatos_results_path)
  File "/Applications/OpenSesame.app/Contents/Resources/lib/python3.7/site-packages/osweb/data.py", line 40, in parse_jatos_results
    for line in fd:
  File "/Applications/OpenSesame.app/Contents/Resources/lib/python3.7/codecs.py", line 322, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xa1 in position 11: invalid start byte