Minor fixes to messages sent from "file in" nodes.

@knolleary, @dceejay, I don't think there are any major compatibility issues with these changes but I'd be interested in your thoughts. I'm trying to write dropbox/s3/box.com nodes and I'd like to get the api clarified.
(Aside: I think it is probably worth reviewing all nodes with respect to this kind of well-definedness as it helps avoid unexpected flow behaviour and simplifies coding of subsequent nodes.)

Specifically:

* in the error case, set msg.filename to be the name of the file used (as
  is done in the non-error case),

* in the error case, delete msg.payload so that subsequent nodes only need
  check for a msg.payload to act upon if they don't care about error cases,
  and

* in the non-error case, delete msg.error to avoid passing through errors
  from earlier nodes to a subsequent node that does care about error cases

Messages sent will now always have well-defined behaviour with respect to
the payload, filename, and error in both error and non-error cases.