One downside is that we keep the wormhole channel allocated longer (we
have to finish the file transfer before we can deallocate it, which
could take a while for large files). Maybe we can fix this in the
future.
Also clean up test_scripts.PregeneratedCode:
* fetch results from both sides at the same time
* only check rc when using a subprocess, since the direct call doesn't
use rc=0 anymore
* no need to cancel the other side's Deferred when one errors
* provide more information if stderr was non-empty
These were split out to make the blocking- and twisted- based
implementations share some code, but now that we're down to just
Twisted, it's clearer to merge them back in.
