runner: Document exit status 14 as possible

Simon McVittierequested to merge
smcv/autopkgtest:document-exit-14 into master

We can report an erroneous package and still have skipped tests.

Signed-off-by: Simon McVittie smcv@debian.org

Noticed while working on https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=901804

Merge request reports