Give better error if instructions are not a valid TSV

[tmtmtmtm]

Several times I've accidentally tried to feed this a CSV file instead of a TSV. In such a case we get a slightly confusing error message:

Traceback (most recent call last):
  File "position_statements.py", line 131, in <module>
    command['property'] = parts[1]
IndexError: list index out of range
<class 'IndexError'>
CRITICAL: Closing network session.

We should spot cases like this (especially as the file needs to have more than one field), and give a better error message.