Partial index promotion due to scrappy spider signals not being handled

I have personally experienced `Ctrl-C` resulting in an incomplete index.

The scrappy documentation for `spider_closed` signal, https://docs.scrapy.org/en/latest/topics/signals.html#scrapy.signals.spider_closed , mentions that the reason for the closing should be `finished` under normal circumstances. However the reason could be `shutdown` in the case of a `Ctrl-C` signal stopping the spider.

It doesn't appear the that doc spider is patching into any of the spider signals. In the case that a spider does not finish naturally the scraper is unaware and will proceed to promote the incomplete temp index.

Inspect the spider signal handling and the temporary-index promotion path described in the issue. Reproduce a Ctrl-C shutdown and compare it with a normally finished spider. Done means a spider that closes for a non-finished reason cannot promote its incomplete temporary index.

Written by the indexing model from the issue text.