[backportpackage] Provide better error message if GPG key is not found

[anatoly techtonik]

$ backportpackage -u ppa:techtonik/backports -s vivid -d trusty mercurial
...
dpkg-buildpackage: full upload (original source is included)
 signfile mercurial_3.1.2-2~ubuntu14.04.1~ppa1.dsc anatoly techtonik <techtonik@UBiX>
gpg: skipped "anatoly techtonik <techtonik@UBiX>": secret key not available
gpg: /tmp/debsign.qzBHNjqh/mercurial_3.1.2-2~ubuntu14.04.1~ppa1.dsc: clearsign failed: secret key not available
debsign: gpg error occurred! Aborting....
backportpackage: Error: debsign returned 2.

It is not clear what to do after this message. The problem is that GPG constructs the name as login@HOST instead of using my <email address hidden>
credentials.

It would be much more friendly if backportpackage proposed to check
your name and email on design 2 error.

[anatoly techtonik (techtonik)]

The hack/workaround to pass this (found after 7 hours) is to supply UBUMAIL in command line:

$ UBUMAIL=`bzr whoami` ./backportpackage -u ppa:techtonik/backports -s vivid -d trusty mercurial

Not a really pleasant user experience.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Dan Streetman (ddstreet)]

please reopen if this problem still exists