BaseEventLoop.call_exception_handler() now accepts a "source" key

BaseEventLoop.call_exception_handler() now accepts a "source" key

[yselivanov, 2014-09-12 22:30:47]

Hm.

I don't like this source / source._source_traceback logic in
`BaseEventLoop.default_exception_handler()`.  Simply because it's an
undocumented attribute of 'context' dict with an undocumented attribute
'_source_traceback'. People may start copying&pasting this in order to have
custom exception handlers.

Can you just call it like this:

     call_exception_handler({
         ...,
         'source_traceback': self.source_traceback
     })

?

Yes, sometimes you'll have Nones in there, but what's the problem with that?

I'd also add 'source_traceback' to the docstring of 'call_exception_handler()'
method.