Verify SPIDER_MANAGER_CLASS interface while loading it in CrawlerRunner #1148
I still prefer this check being done over the complete interface, but we could change it.
Now that I think of it again maybe we could issue a warning instead of an exception if SpiderManager (soon to be SpiderLoader) doesn't comply with the interface, seems like it's the more tolerant alternative.
Another option could be to raise an exception if the absolutely needed methods to run are not implemented (which I think are
I like both options (but the first one slightly more).