add assertion error description for easier debugging

[a-shkarupin]

In case SPIDER_FEED_PARTITIONS setting is set to a value that doesn't match the number of partitions of the spider feed topic in kafka, the following error is reported on the crawler side:

2019-11-29 11:03:22 [kafka.producer.kafka] INFO: Closing the Kafka producer with 0 secs timeout.
2019-11-29 11:03:22 [kafka.producer.kafka] INFO: Proceeding to force close the producer since pending requests could not be completed within timeout 0.
2019-11-29 11:03:22 [kafka.producer.sender] DEBUG: Beginning shutdown of Kafka producer I/O thread, sending remaining records.
2019-11-29 11:03:22 [kafka.conn] INFO: <BrokerConnection node_id=bootstrap-0 host=localhost:9092 [IPv4 ('127.0.0.1', 9092)]>: Closing connection.
2019-11-29 11:03:22 [kafka.producer.sender] DEBUG: Shutdown of Kafka producer I/O thread has completed.
2019-11-29 11:03:22 [kafka.producer.kafka] DEBUG: The Kafka producer has closed.
2019-11-29 11:03:22 [kafka.producer.kafka] INFO: Closing the Kafka producer with 0 secs timeout.
2019-11-29 11:03:22 [kafka.producer.kafka] INFO: Proceeding to force close the producer since pending requests could not be completed within timeout 0.
2019-11-29 11:03:22 [kafka.producer.sender] DEBUG: Beginning shutdown of Kafka producer I/O thread, sending remaining records.
2019-11-29 11:03:22 [kafka.conn] INFO: <BrokerConnection node_id=bootstrap-5 host=localhost:9092 [IPv4 ('127.0.0.1', 9092)]>: Closing connection.
2019-11-29 11:03:22 [kafka.producer.sender] DEBUG: Shutdown of Kafka producer I/O thread has completed.
2019-11-29 11:03:22 [kafka.producer.kafka] DEBUG: The Kafka producer has closed.
Unhandled error in Deferred:
2019-11-29 11:03:22 [twisted] CRITICAL: Unhandled error in Deferred:

Traceback (most recent call last):
File "/home/a/venvs/frontera/lib/python3.6/site-packages/scrapy/crawler.py", line 184, in crawl
return self._crawl(crawler, *args, **kwargs)
File "/home/a/venvs/frontera/lib/python3.6/site-packages/scrapy/crawler.py", line 188, in _crawl
d = crawler.crawl(*args, **kwargs)
File "/home/a/venvs/frontera/lib/python3.6/site-packages/twisted/internet/defer.py", line 1613, in unwindGenerator
return _cancellableInlineCallbacks(gen)
File "/home/a/venvs/frontera/lib/python3.6/site-packages/twisted/internet/defer.py", line 1529, in _cancellableInlineCallbacks
_inlineCallbacks(None, g, status)
--- ---
File "/home/a/venvs/frontera/lib/python3.6/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
result = g.send(result)
File "/home/a/venvs/frontera/lib/python3.6/site-packages/scrapy/crawler.py", line 88, in crawl
yield self.engine.open_spider(self.spider, start_requests)
builtins.AssertionError:

2019-11-29 11:03:22 [twisted] CRITICAL:
Traceback (most recent call last):
File "/home/a/venvs/frontera/lib/python3.6/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
result = g.send(result)
File "/home/a/venvs/frontera/lib/python3.6/site-packages/scrapy/crawler.py", line 88, in crawl
yield self.engine.open_spider(self.spider, start_requests)
AssertionError

It was hard to understand what the cause was. With suggested patch the assertion description is provided in the error, thus making it easier to understand:

2019-11-29 11:04:21 [twisted] CRITICAL:
Traceback (most recent call last):
File "/home/a/venvs/frontera/lib/python3.6/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
result = g.send(result)
File "/home/a/venvs/frontera/lib/python3.6/site-packages/scrapy/crawler.py", line 88, in crawl
yield self.engine.open_spider(self.spider, start_requests)
AssertionError: Number of kafka partitions doesn't match config for spider feed

[sibiryakov]

thank you!