We're storing exceptions captured by Twisted on the media pipeline
cache, but we're also using the defer.returnValue method with our
own methods decorated with @defer.inlineCallbacks.
The defer.returnValue method passes returned values forward by
throwing a defer._DefGen_Return exception, which in its turn
extends the BaseException class and is captured by Twisted.
This way, the latest exception stored in the Failure's object may
also have an HtmlResponse object in its __context__ attribute. As
the Response object also keeps track of the Request object that
has originated it, you could figure it out how many RAM we're
wasting here.
This could easily lead to a Memory Leak problem when running
spiders with Media Pipeline enabled and a particular Request set
that tends to raise a significant number of exceptions.
Example triggers:
- media requests with 404 status responses
- user land exceptins coming from custom middlewares
- etc.
This allows to remove `get_testlog` helper, `flushLoggedErrors` from
twisted.trial.unittest.TestCase and Twisted log observers created for
each test on conftest.py.
