Streams which truncated early with an EOF message would return a
"Failed to authenticate decrypted block" error. While technically
correct, this isn't a helpful error message as it masks the underlying
problem. This changes it to return "unexpected EOF" instead.
The rest of rclone knows it should retry such errors.
This was caused by failing to reset the internal buffer on seek so old
data was read first before the new data.
The unit tests didn't detect this because they were reading to the end
of the file to check integrity and thus emptying the internal buffer.
Both code and unit tests were fixed up.
