Capture exceptions outside of spiders

[maiksprenger]

scrapy-sentry currently does not capture exceptions that happen outside the spider's on_error signal. This limits the usefulness of the extension. Luckily, It is pretty easy to fix as well. I currently have this in my Scrapy settings.py instead of configuring the extension:

from twisted.python import log
from raven import Client

client = Client(dsn=SENTRY_DSN)

def log_sentry(dictionary):
    if dictionary.get('isError') and 'failure' in dictionary:
        try:
            # Raise the failure
            dictionary['failure'].raiseException()
        except:
            # so we can capture it here.
            client.captureException()


log.addObserver(log_sentry)

It should be pretty easy to do the same in the extension, and possibly remove the on_error handling. @llonchj, if you agree, I'm happy to prepare a PR for that.

Above snippet is not my work, it was discovered by @elmkarami in http://stackoverflow.com/questions/25262765/handle-all-exception-in-scrapy-with-sentry and https://github.com/scrapy/scrapy/issues/852.