... (supposedly) some recent dependency change made it so
the (human-readable) error output lands in stderr instead
of stdout (as it should be).
Fixes [ticket:4166](https://tahoe-lafs.org/trac/tahoe-lafs/ticket/4166)
Co-authored-by: Chris Wood <chris@leastauthority.com>
Co-authored-by: Jeff Jacobson <jeffjacobsonhimself@gmail.com>