Move bup index's tick to the correct place; document related risks.

Put bup index's tick *after* update_index(), where it belongs, and
document the risks involved in modifying the filesytem during an index
run, or within too close proximity.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Add the filename/fd and invalid value to stat_struc_to_py error messages.

Thanks to Nathan Bird <ecthellion@gmail.com> for the initial report.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>