PLCImporter loses some ops if an exception is thrown in processing block

[mackuba]

If you use the #fetch method in PLCImporter to process incoming operations, and in the block you've passed, an exception is thrown e.g. on the 400th of 1000 operations in a batch, the importer will break out of the loop, but it will have already updated its internal cursor at @last_date, so the next time it's called, it will continue after the whole batch, skipping the 600 operations that weren't processed.