Currently when an operation with std::fstream in SorterBase::File fails, we check posix error, but don't check fail or bad bits in the std::fstream.

This can lead to error messages like this: 

Error reading file <..>: success 

It would be great to also check fstream for an error code.

However, it still might not be very informative:

code: 1 message: iostream stream error 

But still better than just "success".