[filetransfer] Set the status to error in setException()
FileTransfer would previously not change the status, even though an
exception has been set, leading users to believe that the transfer is
still ongoing, when it is not.
[extensions] Add OutgoingFileTransfer.setCallback()
Some methods of OutgoingFileTransfer take already the
NegotiationProgress callback as argument, some do not. The API is not
ideal, and adding a setter for the callback at least makes it possible
to always have the callback.
