have xfer-client return an error code if it doesn't succeed

[zooko]

This attempt to upload tarballs failed due to insufficient write permissions:

​http://allmydata.org/buildbot/builders/gutsy/builds/582/steps/upload-tarballs/logs/stdio

But the buildstep was green, which led to the problem not being discovered for a while.

It looks like xfer-client might not be returning an error code for this case.

[warner]

I've replaced the old xfer-client with a modern flappclient call (in eff82174d50cb326), and set up a corresponding flappserver in the 'source' account on allmydata.org . We still need to:

• replace the gutsy buildslave (which is where the tarballs used to be generated and uploaded)
• copy the flappserver's FURL into the new buildslave

But since flappclient properly returns an error code for failed uploads, even when the target directory has lost write permission after the flappserver was started, I'm now closing out this ticket.