tmpfail results lead to lost test requests

In some cases, test requests end up tmpfailing even after retries, like

https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-utopic/utopic/i386/f/freeradius/20150714_123217@/log.gz
https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-utopic/utopic/i386/libr/libreoffice/20150714_101004@/log.gz

These get ack'ed from the queues, but britney does not pick up the result. Thus excuses.html stays "in progress" and britney still thinks they are pending:

$ cat ~/proposed-migration/data/utopic-proposed/autopkgtest/pending.txt
freeradius 2.1.12+dfsg-1.3ubuntu2 i386 mysql-5.5 5.5.43-0ubuntu0.14.10.2
libreoffice 1:4.3.7~rc2-0ubuntu1 i386 mysql-5.5 5.5.43-0ubuntu0.14.10.2

The log has:

10:08:37.log:E: [Tue Jul 14 10:13:02 2015] - https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-utopic/utopic/i386/libr/libreoffice/20150714_101004@/result.tar is damaged: "filename 'testpkg-version' not found"

14:51:32.log:E: [Tue Jul 14 14:57:16 2015] - https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-utopic/utopic/i386/f/freeradius/20150714_123217@/result.tar is damaged: "filename 'testpkg-version' not found"

This should count as a test failure for now. Later we might fix the underlying issue of the tmpfail, but we should never lose pending tests that way.