InvalidTableIndex error doesn't match input index
InvalidTableIndex error doesn't match input index

The index value is modified in-place, leading to a misleading error message. For example, [here](https://github.com/python-hyper/hpack/blob/master/test/test_table.py#L36), the message reads `Invalid table index -63`, rather than `Invalid table index 0`.
python-hyper/hpack,python-hyper/hpack