5752 rescue parsedfile prefetch errors

This is a fixup of GH-1100.

If a read error occurs while the parsedfile provider is prefetching targets, it will propagate that error up to the transaction and will not prefetch any more records. This has the side effect of wiping out all unmanaged resources that haven't been prefetched. This set of commits explicitly handles read errors in the parsedfile provider to rescue read errors and try to prefetch as many targets as possible.