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 `skimage.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 PR fixes the problem - which was the only current Python 2.6.x anomaly I'm aware of. 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.