Bugfix: issue #6

- Use well defined error code to determine whether an AddChunk has
  failed or another client has added the chunk.
- Change the return type of Client.findChunkLocations and
  Client.addChunk because they essentially return the same things and it
  would simplify the error handling code, since we don't need to define
  two type of replies for those two function calls.
Errorcodes.go for error code definitions