Clean up batch vs single error handling

https://github.com/google/apitools/pull/145

So, this change further increases the delta between error handling in the batch and non-batch case... after discussing with @craigcitro, in the short term we can probably update BatchHttpRequest#_Execute to use each request's check_response_func
https://github.com/google/apitools/blob/master/apitools/base/py/batch.py#L409
and in the longer run, we should further unify the code of a batch and non batch request (and the error handling)