Throw a meaningful warning when package management is busy

[fossfreedom]

When installing applets/themes and other software, welcome fails to install due to the package management system being invoked else where e.g. automated updates

341 packages can be upgraded. Run 'apt list --upgradable' to see them.
E: Could not get lock /var/lib/dpkg/lock-frontend - open (11: Resource temporarily unavailable)
E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), is another process using it?
[Apt] Reloading cache... 
[Apt] Cache reloaded. 

We should test for this condition and ask the user to try later rather then throw a fail message.