Do not silence download errors when request errback raises an exception
Do not log IgnoreRequest exceptions as download failures
engine multispider support removed
Merge pull request #418 from nramirezuy/engine-multispider

engine multispider support removed
Merge pull request #380 from dangra/dont-log-ignorerequest

Dont log IgnoreRequest exception as download failure
DownloaderMiddleware docs: Update process_request, proper explanation of IgnoreRequest, and some other minor cleanups.