info: remove an unused variable

[endlessm/eos-shell#4147]
info: allow to check for updates from error state

If we detect an error state when the panel is launched (i.e. not as a
consequence of us calling into the OSTree daemon), make it possible to
check for updates by callinh poll() again on the daemon, like
eos-updater does.

[endlessm/eos-shell#4147]
info: show a message when updates fail

Instead of just hiding the widgetry.

[endlessm/eos-shell#4147]
Translations for failed update message

To take advantage of the existing translations for the
update error in eos-shell, we changed the source string
from "Error checking for updates" to "Update failed".

[endlessm/eos-shell#4147]
Merge pull request #27 from endlessm/shell/4147

Shell/4147