FIX: Specify field index for Python 2.6 compatible use of str.format()

For people who may be stuck with production systems running Python 2.6.x, the novice module was regularly failing four tests because the formatting of the string when going to `raise IndexError` in `_verify_picture_index` was itself raising a `ValueError: zero length field name in format`.

This is the only current Python 2.6.x anomaly I'm aware of, and this PR fixes it. The solution is to explicitly specify the index when using `str.format()` - this is required in Python 2.6 but optional in all future versions.

However, 2.7+ happily runs with the explicit field name, so this fix is fully compatible.