Add dcraw exceptions

[OhmGeek]

While we, in comments, have printed the error stream from the DCRaw process, it would be good to look at the stream and report errors:

If the stream is null, then there is no error. That's all great. If there is an error, we will need to create a new exception (perhaps called it DcrawNativeException), with the message inside being the message output from the stream. This message can be retrieved using a BufferedReader.