dwke - Skip section if Section dir is inaccessible.

detect_well_known_errors() was crashing during the daily run on days
when a section was added (due to a return with the wrong number of
arguments).
p-s: skip upgrade tests involving downgrades

Report these tests as untestable.
Downgrades 'happen' e.g. in stable2bpo2testing if the version in backports
is (temporarily) newer than testing.

Signed-off-by: Andreas Beckmann <anbe@debian.org>
Workaround #709372 and ignore broken symlinks found by adequate.