This happened when the underlying reader returned io.ErrUnexpectedEOF.
The error handling for the call to io.ReadFull failed to take this
into account.
io.ErrUnexpectedEOF is reasonably common when SSL connections go wrong
when communicating with ACD, so it manifested itself as transfers from
non-encrypted ACD to encrypted ACD getting stuck.
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.
