intecept.py: Return SERVFAIL if upstream doesn't respond
intecept.py: Return SERVFAIL if upstream doesn't respond

This allows tests to differentiate between legit NXDOMAINs and
upstream being slow or broken.

Signed-off-by: Nolan Leake <2b78f1a11b050030c931d98cf25a8c2d3b85b913@sigbus.net>

paulc/dnslib,paulc/dnslib