Report the amount of input inspected on failure
Currently, we report the amount of input consumed on parse success but nothing on failure.
We could be a little more helpful by reporting, on failure, the furthest input position that was visited by the parser. Note that this would not be the length of a "partial success" or "how far we got" (we don't know these things), but literally how far we had ever looked at the input (in any branch) before determining failure.
Thus it would provide at least an upper bound on the location of an error in the input.