More transparent handling of CastErrors

[micimize]

I think exception handling should be done at the processor level, but just giving a way to override the behavior would be good as well

dataflows/dataflows/base/datastream_processor.py

Lines 82 to 90 in 5f4aeff

	def process(self):
	ds = self._process()
	try:
	for res in ds.res_iter:
	collections.deque(res, maxlen=0)
	except CastError as e:
	for err in e.errors:
	logging.error('%s', err)
	return ds.dp, ds.merge_stats()

[akariv]

@micimize
Note that the validate processor provides means to act upon individual casting errors (including whether to drop/pass the failing rows). You can add this processor as the last one in your flow to create special handling for such errors.
I think the error logs in the process method should remain - as an indication to the user that something is not right - but the handling should be done using a processor.