Should be more verbose with unsatisfiable dependencies

Forwarded from http://bugs.debian.org/473542.

jon@ANKH:~$ sudo gdebi swiftweasel-2.0.0.13_pentium-4_ubuntu-i386.deb
[sudo] password for jon:
Reading package lists: Done
Reading state information: Done
Reading state information: Done
Reading state information: Done
This package is uninstallable
Failed to satisfy all dependencies (broken cache)jon@ANKH:~$

It would be handy to know what was wrong with the cache and
how a beginner might go about fixing it.

In this case,

jon@ANKH:~$ sudo apt-get check
Reading package lists... Done
Building dependency tree
Reading state information... Done
You might want to run ‘apt-get -f install’ to correct these.
The following packages have unmet dependencies.
  gnome-do: Depends: libcairo2 (>= 1.5.12) but 1.4.14-1 is installed
            Recommends: gnome-do-plugins but it is not installable
E: Unmet dependencies. Try using -f.