Minor fixes to messages sent from "file in" 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.
Merge pull request #437 from hindessm/file-query-node-usability

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