[MRG+1] add item_error to be catchable

[chainly]

add item_error to be catchable (ie. by extension with signal).

[lucywang000]

@chainly can you explain the motivation for this, like a typical use case to justify this new signal? What's more I think a new test case is required.

[codecov]

Codecov Report

Merging #3256 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3256      +/-   ##
==========================================
+ Coverage   82.12%   82.15%   +0.03%     
==========================================
  Files         228      228              
  Lines        9599     9601       +2     
  Branches     1385     1385              
==========================================
+ Hits         7883     7888       +5     
+ Misses       1457     1454       -3     
  Partials      259      259

Impacted Files	Coverage Δ
scrapy/signals.py	100% <100%> (ø)	⬆️
scrapy/core/scraper.py	88.51% <100%> (+2.11%)	⬆️

[chainly]

I want to count/know how many item dropped by unhandled exception occurred in pipeline easily , but only printed in log.

[kmike]

This feature makes sense to me, and implementation looks good. The only question I have is why is Failure returned, not an exception, like in item_dropped?

[chainly]

1. There are mistake in test case(self.run.itemerror ==> self.run.itemresp), now fix. Thanks to the question : )
2. For the question: if someone need traceback, Failure a good choise, see the doc failure.Failure, and failure.Failure.getTracebackObject. Why lost, not known.

# test code
def item_error(self, item, response, spider, failure):
    import traceback
    print('exception')
    traceback.print_exc()
    self.itemerror.append((item, response, spider, failure))

def _assert_items_error(self):
    self.assertEqual(2, len(self.run.itemerror))
    for item, response, spider, failure in self.run.itemerror:
        self.assertEqual(failure.value.__class__, ZeroDivisionError)
        import traceback
        print('failure')
        traceback.print_tb(failure.getTracebackObject())
        self.assertEqual(spider,self.run.spider2)

# output
--------------------------------------------------------------------------- Captured stdout call ----------------------------------------------------------------------------
exception
exception
failure
--------------------------------------------------------------------------- Captured stderr call ----------------------------------------------------------------------------
NoneType: None
NoneType: None
  File "/Users/xxx/anaconda/envs/python36/lib/python3.6/site-packages/twisted/internet/defer.py", line 653, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/test/ttttt/scrapy/tests/pipelines.py", line 17, in process_item
    1/0

[whalebot-helmsman]

@chainly Problem with unit-tests was fixed in master. Can you merge master to this PR?

[chainly]

OK, fast-forwarded and push again.