Feeding tables with fewer columns than expected to importer tracebacks

[soininen]

I had an Importer specification that expected to process a five-column csv table but then I changed the file's layout to contain only four column. Trying to import the file then resulted in Importer hanging for eternity and this Traceback:

Traceback (most recent call last):
  File "lib\multiprocessing\process.py", line 315, in _bootstrap
    self.run()
  File "spine_engine\utils\returning_process.py", line 50, in run
    return_value = self._target(self, *self._args, **self._kwargs)
  File "spine-items\spine_items\importer\do_work.py", line 64, in do_work
    data, errors = connector.get_mapped_data(
  File "spinedb_api\spine_io\importers\reader.py", line 142, in get_mapped_data
    data, t_errors = get_mapped_data(
  File "spinedb_api\import_mapping\generator.py", line 108, in get_mapped_data
    mapping.import_row(row, read_state, mapped_data)
  File "spinedb_api\import_mapping\import_mapping.py", line 264, in import_row
    self.child.import_row(source_row, state, mapped_data, errors=errors)
  File "spinedb_api\import_mapping\import_mapping.py", line 264, in import_row
    self.child.import_row(source_row, state, mapped_data, errors=errors)
  File "spinedb_api\import_mapping\import_mapping.py", line 264, in import_row
    self.child.import_row(source_row, state, mapped_data, errors=errors)
  [Previous line repeated 3 more times]
  File "spinedb_api\import_mapping\import_mapping.py", line 246, in import_row
    source_data = self._data(source_row)
  File "spinedb_api\import_mapping\import_mapping.py", line 269, in _data
    return source_row[self.position]
IndexError: list index out of range

This happened in master but we need to check if the bug is still present in 0.8 branch.