Log unhandled errors during perform

[radix]

Unlike Deferreds, Effects can't have callbacks added to them after a result is available. This means that they have a handy property that once an effect has been performed, and it resulted in an error, we can know that that error won't be handled by any other code, and log it to stderr.

ideally the top-level of your program is:

from effect import perform
...
if __name__ == '__main__':
    perform(dispatcher, main())

in which case having a default logger seems like a pretty reasonable solution. But if you're using effect.twisted.perform, since it provides the result in a way other than callbacks to the effects (by returning it as a deferred), there is still a chance to add error-handling errbacks.

So how about this: effect.perform() could default to logging errors, since it's otherwise impossible for callers to report unhandled errors. effect.twisted.perform and effect.sync_perform should not have this behavior, and just have the same behavior they have now.

[tomprince]

Note that effect.twisted.perform and effect.sync_perform never pass an effect that results in a error to perform (since they add error handlers).