athena-auto-update should check for being unable to acquire the apt lock

[jdreed]

athena-auto-update occasionally ends up fighting with the cron-invocated "apt-get update", such that it fails with:

Running apt-get install
** Running: apt-get --fix-broken --assume-yes install
E: Could not get lock /var/lib/dpkg/lock - open (11: Resource temporarily unavailable)
E: Unable to lock the administration directory (/var/lib/dpkg/), is another process using it?
ERROR: apt-get install failed, but continuing anyway
** Running: dpkg --configure -a
dpkg: error: dpkg status database is locked by another process
ERROR: Failed to configure unconfigured packages.
** Ending Athena auto-update at Tue Mar 11 07:35:01 EDT 2014

This should be a soft failure, and it should just try again later. Or at least it should update the status message to something different. We could also disable whatever cron job does this, too.