Postprocessing plugin errors: print them to stderr too

Postprocessing plugin errors: print them to stderr too

Failures to run the postprocessing plugin are currently mostly ignored
- they result in a message in the slicing engine log, but the slice is
considered to have succeeded.

Ideally slicing engine failures, and plugin failures, would be treated
the same way.  But currently engine errors result in a message on
stderr and the slice simply not finishing in the GUI - that is, there
is not even any way to view the log.

Ideally there would be a `slicing failed' state for the slicing
button, as well as `not yet finished'.  That way the user could know
that something was wrong, and know to look at the log.

For now, however, just print the error message to stderr with
traceback.print_exc, too, so that at least command-line users get to
see something.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>