Expected 1677216 but read 8441856.

It's interesting that you can't apparently get any error information (other than "there was an error") after fread. Maybe dcp::File should use read() / _read() so we can get more information. I still have this nagging feeling that sometimes we should retry in cases like this, but I can't find anybody on the internet to confirm that (most people say fread errors are always unrecoverable)