fix: always validate PrepareManager cache in pipeline mode

fix: always validate PrepareManager cache in pipeline mode

Previously, when processing results in pipeline mode
(BasePipeline._process_results()), we'd run
'cursor._check_results(results)' early before calling
_prepared.validate() with prepared statement information. However, if
this check step fails, for example if the pipeline got aborted due to a
previous error, the latter step (PrepareManager cache validation) was
not run.

We fix this by reversing the logic, and checking results last.

However, this is not enough, because the results processing logic in
BasePipeline._fetch_gen() or _communicate_gen(), which sequentially
walked through fetched results, would typically stop at the first
exception and thus possibly never go through the step of validating
PrepareManager cache if a previous error happened.

We fix that by making sure that *all* results are processed, possibly
capturing the first exception and then re-raising it. In both
_communicate_gen() and _fetch_gen(), we no longer store results in the
'to_process' like, but process then upon reception as this logic is no
longer needed.

Fix #585.